Mysterious magical items are neat. Every single magical item being mysterious is a pain. And really, the magic items/wealth system is just an XP system for items anyway. If characters are expected to use magical items in accordance with their level, and certainly within 3&4e they are, then it makes no more sense to make items more mysterious than spells or feats unknown because they aren't common or forcing all races to only appear in the MM.
This isn't to say that no magical items should be weird and unknown. Artifacts fit that niche, and 4e has done a good job of making artifacts more than just magic items with more power than the wealth rules allow.