Because you're making it sound as though the benefits are all one-way, which isn't the case at all.
Any player we bring in on loan is obviously recruited on the basis that our manager believes he's better than anyone we currently have, or could get otherwise, to play that position. A better player means a better team, which hopefully means a higher league placing, which means more money (although apparently the extra money from league placings doesn't really matter to some fans).
Does having a player out on loan benefit the parent club? Of course it does, otherwise what's the point in them doing it? Everyone benefits theoretically. The player gets game time, the parent club sees their player get some experience, and the loan club gets a player that the manager believes will improve the side.
What I am against is loaning in absolute fucking lumps like Conor Sammon. If a potential loan player isn't good enough to play for the first team and add some quality that we don't already have then there's no point in doing such a deal.
But if Celtic, Rangers, or any other club for that matter, are looking to loan out a player and our manager thinks "yeah, that lad could do us a real turn. He's of the quality we'd likely never get otherwise and he can improve our team" then I'd have no issue at all with the manager sounding the player out.