Here is a great example of this. Granted, that was that user's first post, but this is still common.
UPDATE: That example has since been updated, so consult the original version.
This type of answer isn't always favored on SO, either. Even if such answers may benefit the OP, it may not benefit visitors that don't fully understand the code and/or the specific context. I think this is even more detrimental for CR because the intent is to review, not simply to give (after all, we do not write code on demand, but we can at our discretion). For instance: if the question specifically asks for ways to simplify the code, not everyone may understand why (or if) this answer works. I wouldn't prefer to consider votes the only measure of validity, though. If that were the case, then anyone (that is, someone not experienced with that language) could think, "this definitely looks shorter!", and upvote it. What if it happens to be flawed (without criticism from more experienced users), and the answerer didn't "argue" his/her case in the form of an explanation and/or code comments?
Anyway, back to the issue at hand. Should this be a requirement? Specifically, should such posts receive comments telling the answerer to explain the code in some way? The voting is, of course, up to the individual, but I personally do not consider such an answer worthy of a downvote in and of itself.