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...