It's my belief that loaning players is not in their best interest as it puts their fate in the hands of another manager who knows he'll lose him at the end of the spell anyway. You don't see Wenger or Ferguson loaning youngsters out all the time, and when they do the players clearly don't really have a future at the club.
Loaning and co-ownerships should be banned outright as they are unfair to smaller clubs and sometimes stifle the development of younger players.
NO to loaning and co-owns.
I don't like loaning, as a general rule, and I would ban co-ownerships.
However, there are occasions when loaning is beneficial to all parties, so I don't see any problem with some youngsters going on loan.
Players who are 17-19 shouldn't be loaned, they should develop through proper coaching at the club.
Players who are 20-22, but aren't ready for the first team or will struggle to get playing time, should be loaned out because that is an age at which development can stagnate. However, there aren't many players who aren't ready at 21 or 22 that go on to be very good players.
So I agree that loaning isn't usually a good thing, but, in certain cirumstances, it can be the best option.