I think these lads deserved first-team experience outside of United.
Remember Jonathon Greening and Mark Wilson? They are also in United's reserve a few years back but when McClaren signed as Boro's Manager, the first thing he did was to sign these 2 fellows.
Greening was doing reasonably well back den at United and he had fine spells with Boro and West Brom but he's off the radar now. Wilson never make it big and disappeared as well.

It's better to cash in than lose them for nothing. You know what i mean.

Cheers.
CK