My two cents:
I reckon there's a lot to be learned in multiple-session redpointing. Due in part to logistics and in part to preference, I almost always find myself going for onsights or quick ticks when I'm out climbing. However, when I put a bit more time into something, I find myself picking up "subtleties" of movement that would be lost amongst the new stimulus of trying a route from scratch. Pushing a bit more with this foot, catching this hold slightly differently - there's a lot more trial and error (and hence, learning) than in onsighting.
Therefore, I would argue that sieging stops being useful for progression once you stop changing things and are just waiting for the perfect go. At this point, I would argue it would be more beneficial to move on. Once you're stronger/better, you might be able to learn more from the siege route.
Disclaimer: I haven't climbed anything recently that's more than a grade harder than my max onsight.
