It might be completely unremarkable if there were only a couple of such names on this server, but lately there are many people now going by monikers such as user3432 and user5633, who've accumulated a fair bit of reputation. Here are a couple of reasons why I think continuing in that way is bad:
I like associating names to identities, as certain posters have distinct expertise. Suppose you're looking for good answers on say, set theory -- you might decide to ready daily some of User A's answers. Or if you're looking for number theory, you might to decide to read all of User B's. Names that don't vary much from each other, like "UserXXXX," make it hard to remember the identity of posters you want to keep track of.
Related is that "userXXXX" names make it hard to search. If one person's username differs from another person's name in only a couple of places, one has to needlessly memorize an arbitrary 4 or 5 digit string just to locate a unique user. Why? Everyone wants to be read and sought out; keeping "userXXXX" makes that harder for essentially no reason.
Allowing default usernames encourages bad behavior, by muddling identities. For example, I know quite a few people who blog on Blogspot who will not allow comments to their posts with the default "Anonymous." They force everyone to choose a unique name. Why? One reason is that some people will nefariously hide behind "userXXXX" type usernames to make spam posts that will not be immediately recognized for flagging. What's more important here is that simply picking out a username and assuming a unique identity is an investment in of itself, and signals that one wants to become involved, and is not simply a passive member of the community. It demonstrates engagement.
I understand the need some feel to be relatively anonymous; that's fine, I too like to keep many of Internet identities separate from one another. And, choosing a unique username might seem flippant and unimportant matter compared to the main mission of the site, asking and answering questions. But if that's true, the answer isn't to stick with "userXXXX" -- better to go to a goobledygook concatenation of words or something than to stick with that.