I would also suggest that it depends on whether we have signed him as Toby's replacement (ie we see him as Fonte's regular partner), or if we have signed him as a result of Gardos' injury. If it's the latter, then a loan makes a lot of sense. Gardos (hopefully) wont be injured forever!
Loans fulfil a need because they can be a quick fix when you lose a player (like Gardos), but sometimes they are done because of doubts on one or both sides. And (though probably not in Caulker's case) a player may be loaned out because he isn't for sale at that point as with Gallagher and Toby. Can mean you get a player you would like to keep (but can't), but you also get to try before you buy (wish Osvaldo had been on a loan ). Loans can work out and purchases may not (and vice versa). The only danger is we can sprinkle Saints stardust on a player just to lose out.
I like it and think loans can work. Toby was a success - we just need to pump up the 'buyback clause' this time so it's not viable for QPR to recall him if he has a good season.
I have often referred to sprinkling Saints stardust on players and I note that Les Reed is using the same term....talked about Saints dust on players.