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

It's just ranking, you equally have the possibility to narrow down by keywords with either sorting method.

Honestly, if your interested about the feedback of a CPAN Modul X::Y and you get 100 hits, wouldn't you preferre to start reading the 10 posts with rep>20 before getting to rep=0 ???

Of course there are much better and sophisticated ranking methods possible with the metadata we have¹ ... but sorting by rep is much simpler to implement just by extending an SQL-Query. It's easier to get the break even of investment and payback.

(¹) like weighting the votes by XP of the voter, or by "voters who voted like you".

UPDATE: Or weighting by voters who regularly vote for "too-late-posts" (posts which slipped out of focus of average voters).