Can anyone provide a strong argument for noun-adjective agreement in number i.e. for attaching plural adjectives to plural nouns?
I am fine with noun-adjective agreement with respect to case, and also with plural adjectives by themselves. It is simply the mandatory use of the plural adjective with the plural noun that has never appealed to me. I've heard, and sometimes made, the following arguments:

(1) Flexible word order requires some degree of redundancy. An example is given by Piron (I am paraphrasing here):

"We support the amendments to the resolution proposed by India."

Piron points out that the choice of propinita or propinitaj lets the audience know whether it was resolution or the amendments that India proposed. This argument has never convinced me. The redundancy only helps with a comparatively small set of sentences in which one thing and several things are juxtaposed. If there had been two resolutions or just one amendment, noun-adjective agreement would not have supplied any clarification. Another argument is

(2) It is easier to shift endings for several consecutive words than for a single word.

Maybe. I tried this out in Russian, and it seemed to be true. But this is probably due to the fact that one learns to speak this way in Russian, and not to intrinsic ease.

(3) Since the adjective often stands in for the noun, it is a good idea to have plural adjectives. So, for the sake of consistency, adjectives modifying plural nouns should always be plural.

Okay, if consistency is the only goal. If you could forgo a little consistency for the sake of logic and economy, you could allow the adjective to remain singular except when it stands in for the noun. The last argument is

(4) A plural noun simply "should" have a plural modifier. Foreign friends have told me that English adjectives seem "bare" to them.

To me, this is not really an argument, but a statement of taste and aesthetics.

Has anyone another argument? Or a reason to to be convinced by any of the forgoing? Thanks, and apologies for the length of the post.

Personally I've never quite understood why people make such a big deal out of the whole adjectivial agreement thing (I'm not saying that's what you're doing by the way, just speaking generally). Esperanto has several other more problematic flaws than adjectivial agreement, something which for most people becomes second nature in fairly short order. Yet for some reason this rather innocuous feature more than any other causes people critical of Esperanto to spew forth endless bile and vitriol. Very strange.

That aside, I think you're right in that the arguments in it's favour don't stand up from the point of view of pure simplicity, logic and economy. You probably already know that Zamenhof himself proposed to get rid of it, describing it as "superfluous ballast".

I myself quite like the way the concordant "aj"s and "oj"s impart a particular aesthetic to the sound of Esperanto, which I find quite pleasing.

Concerning adjectives, it's not that bigger deal to me (I personally would have preferred a different plural system altogether as I find it hard to hear the difference between o and oj - Ido is a little easier to understand audibly for me in that respect), however I think if you get rid of the plural for adjectives you should get rid of the accusative too. Whether you keep the accusative on the noun and simply take it away from the adjective, it doesn't make a difference in my mind. I just don't think that it would be wise to take one away and not the other - they seem to compliment each other to some extent.

Sure you could have a language without, but not Esperanto because the agreement is intrinsic to the language. I personally like it, and it seems obvious to me that if case agrees then so must number. But maybe there is no very good reason for it.

The simplest justification is that agreement in number is evident from the untouchable Fundamento (Section 7, 2nd example and section 11, last example). I think Rogir has made an important point: if you feel that in this or that respect the language should have been differently designed, and you change it accordingly, then you no longer have Esperanto but some other language. That is how Ido came into being.

Zamenhof was coerced in 1894 into proposing "improvements", and the Esperanto community rejected them. The next generation of "improvers" attempted deception. Once they were found out, the answer was 'No' again.

I suggest that you read these two chapters from the book "Lingvo kaj Vivo" of Gaston Waringhien [translated into English by Don Harlow], justifying the usage of the accusative ending [the one-half of the noun-adjective agreement].

The Accusative in Interlinguistics: http://donh.best.vwh.net/Languages/akuzativo.html

The Accusative in Esperanto: http://donh.best.vwh.net/Languages/akuzativo2.html

Miland:The next generation of "improvers" attempted deception. Once they were found out, the answer was 'No' again.

What's this deception? I've heard about it but never quite figured out what people point to when referring to it. Personally I don't see how deception works in with language development, to me it's like saying the French deceived the English into using defective counterfeit latin roots made using lesser quality materials thus making English inferior in quality - thus my confusion

Iĉo:I'm trying to learn to love it.

Give it time.

I think we Aussies are less likely to like the esperanto plural because it probably reminds us of Kath and Kim or some other over-exaggerated Australian accent

"Oi, kim, now look at moooooooi...."

That said, I swear my 'right' sounds like 'roite' (with a US 'o') more and more everyday.

ceigered:What's this deception? I've heard about it but never quite figured out what people point to when referring to it.

The deception has nothing to do with the language itself, or vocabulary choice or anything like that.

What happened was that there was going to be an international committed that would hold a meeting to choose which was the best international language, of the many projects of the time, so that international organizations could support the best one. Each language sent a representative to the meeting, to make the case for that language. A certain person was sent to represent Esperanto at the meeting, a longtime Esperantist, trusted by Zamenhof and by the community. Long story short - he didn't represent Esperanto at all. The man was Louis de Beaufront, co-author of Ido. Instead of being a proponent of Esperanto at the meeting, as he was meant to be, he proposed that the international language should be an "improved" Esperanto - Ido! He denied having been involved in creating Ido, and he denied having stabbed Esperanto in the back at the meeting. Oh, and by the way, the leader of the delegation to choose the language? Couturat, the other co-creator of Ido, who had previously assured Zamenhof that no problem, the committee would definitely choose Esperanto.

You can read about it in greater detail on Wikipedia

Can anyone provide a strong argument for noun-adjective agreement in number

Yes.

Redundancy is the argument. Redundancy in the sense of including the same information more than once in a sentence, so that if the sentence is misheard or miswritten, the important information will still get transmitted, because it is being transmitted in two ways and not just one.

This could be considered important for something like an international language, where people with different native languages and different accents are communicating; sometimes a little redundancy helps in ensuring the point gets across.

Even in English we have some degree of redundancy. Verbs change form for singular or plural (depending on which pronoun), so the pronoun and the verb form both give information on singular or plural subjects.

The same rationale works for the accusative. Word order provides a hint about what the object is - and the -n ending emphasizes it. So you have two pieces of information, not just one, to ensure that you have understood correctly.