If I want to do things on Wikipedia from Lynx [0] I should be able to do as much as Lynx supports, not less because my useragent doesn't match something that we support.
Also if we cut lynx support Jidini will come and kill us in our sleep. Dead developers = productivity loss ;)
----- On a serious note, I agree with what others have said about making sure that there is a link that you can click and get to an edit form that will submit on essentially all browsers is not hard and something that should always work. More fancy features don't need to work on all browsers. Personally doing something like support last two latest versions of browser X doesn't really make sense to me. We should support what people use. If no one uses latest and greatest browser X, we shouldn't bother with it. If (a significant number of) people are still using firefox 3.5 for some unknown reason, we should still support it. Thus I think we should stick to using percentages, perhaps with varying levels of support for different percentages. I'm really unclear on what benefit we would get from declaring a list of supported browsers [that are somewhat unrelated to viewership percentages] instead of directly looking at the user-agent statistics.
-bawolff