Is it?
Developing players is all about playing against players who are as good if not better. Look around the under 21 squads from 2013 (just 1 year ago) and there are several players who could easily walk into any Premier League team. Even some of them were playing at the Euros the year before.
I think so, yes.
A Champions League game in front of 60,000 will teach them far more than some U21 against weaker opposition.
If we have U21s not getting experience then they should be play, but the likes of Sterling really arent going to learn a lot.