I mean, what's the alternative? Even if it's nowhere near perfect, it's far, far better than no information at all. Surely you'll concede that there's an objective difference between something that will take one person 1–5 days, vs. a team 6–12 months?

Also, the process of estimation very often reveals difficulties or unknowns that nobody was even thinking about previously. That's the whole purpose of estimation happening via unanimous planning poker -- the discussion as to whether a feature is a 3 or a 10 is incredibly valuable in getting on the same page as to what the feature even is.