I'm guilty of indulging in scriptaculous, and I believe that, if used well, it is worth the extra downloads. I'm thinking just for AJAX comments, you could do the same thing with about 1/10th of that file (most of the effects, etc you do not use).
However for pages with more extensive use of AJAX/JS in general, the full library would be justified. Pages that, IMO, are justified for prototype (and such) are pages with many tedious fields to fill (password...) like signup forms, where the user doesn't want to wait for the page to reload to know that the username is took, or that their passwords do not match.
I just spent a good bit of time building a custom simple CMS system, and I used a good bit of AJAX (Prototype / Scriptaculous) in the back-end interface, and it has made a huge difference in usability.
You have to think that your users aren't really worried about how much data they're downloading. They're thinking about time. So a page with some strategic AJAX controls may take a few extra seconds to load initially, but the way AJAX lets users continue to work on other parts of a page while form data is submitted actually saves them time.
I've definitely seen AJAX misused, but I think that if it is used wisely, it can make life easier on your users, and that's the point.
The correct and proper use of AJAX (and the recognition of using such a combination of technologies) was indeed a worthwhile step forward (introducing a number of backward stumbles as well!) for the web and it would be silly to ignore its place in today's web technology sphere. That said, perhaps the original poster could do with a little sideways thinking (or, indeed, directly under your feet thinking) since the main cited problem against using AJAX is simply that one "must" (over)load the web site with large libraries. The solution? Just don't use those large libraries! (Not really an inspired piece of advice but worth making note of.) There are countless alternatives (including the old home-brew approach) available to provide a firm base for rapid AJAX development which might require a couple of kilobytes or less cf. those larger frameworks.
If you really want to make use of the larger frameworks, there are a whole heap of performancing steps which can be undertaken to help reduce some of the problems encountered in the past (large library size included) if you're interested.
I tend to think that AJAX is overused. Certainly it has it's place, and can make real improvements to usability. But, I think it's just become another buzzword, and people throw it in anywhere they can. I've actually had clients say they want AJAX, even though they don't know what it is.
Yeah, I don't think it's necessary that you have AJAX on there. But then again it really depends on what the website is for and who your target audience is. If this audience is someone who is used to seeing and using websites with AJAX then you might consider it.