But I'm pretty confident that sorting by reputation can be "more useful" than sorting by time.
I wouldn't be so confident :) Actually, I think using other means to narrow down the search (additional keywords, etc.) such that sorting of the remaining hits becomes more or less irrelevant, is the better strategy.
And if it's not possible to search by rep, why do we bother to collect them, only to keep people busy and interested in the XP-game???
I'd say, yes, it's the game, and its - hopefully beneficial - side effects (others might disagree, though). Or maybe somewhat more favorably put: rep can give some kind of community feedback, if you factor out the counterproductive influences (exposure, etc., like I mentioned above) for yourself...