Of course, I agree that we shouldn't field a weakened side at all but if we do, I think we should do so against Barca rather than Watford.
It's highly unlikely that we'll progress in the CL, so it would make sense to try and get the FA Cup for a 3rd year running, no?
Let's progress in the FA Cup first and play a near enough full 1st team and see how we're doing squad wise after that. There does seem to be little point in risking too much against Barca only to have this impact the league and cup run ins. Got a tricky away tie after the Barca game to deal with as well and the squad's not very big at the moment.