There are actually tangible rewards: multiple people in this thread have talked about how they've had discussions with recruiters who examined their activity history and commit streaks. So maybe having longer streaks helps you get a job, and maybe having shorter ones hurts. It's anecdotal, yes, but it's not intangible.

But I think your second paragraph addresses the real problem: contributing 18hrs/weekday and contributing 5mins/day are both legitimate open source contribution styles. But the "streaks" metric removes all subtlety -- how many contributions are they actually making; are the changes meaty or trivial; do the changes break CI; do the changes address Issues in their respective repos -- and so I don't believe it is a helpful metric to display.

Personally, I believe it does more harm that good, by encouraging people to maintain streaks even if doing so actually decreases their true productivity or happiness. But no matter how much or how little harm it does, I think it is hard to argue that it does much good, since it is such a high-level distillation of complex information.