Loans are one thing, co-ownerships are a total other thing.
Loans make sense, as they are a good way to get young players experience, when they might not get it otherwise. Its a fairly risk-free way to get your prospects some game time.
Co-ownerships are just ridiculously stupid. Selling a 50% share in a decent player... if they turn to shit, your 50% is worth nothing... if they turn good, as was the case with Chiellini, Miccoli, Criscito and now De Ceglie, one has to pay a premium to get them back... typically, as high as the other team can possibly go.