I'd say that's more down to circumstances and the individual players; I'd judge each case on its merits and what the player can offer. Being the size of club we are, it's inevitable that we'll tend to have players on their way up, or on their way back down again. It therefore follows that the 'return' visit is likely to be disappointing. The exceptions are those where someone might have left at a young age and since turned into a better player - Paul McGinn, for example.
In Naismith's case, his injuries have obviously hampered his career. He was out for a long spell before leaving us, and has clearly wrecked his chances elsewhere. If he appears to be over it, I'd be happy to take him back. To be fair, if County stay up, he may not want to leave; he's always spoken very fondly of his time in the Highlands and that was obviously a factor in going back there.
He's only 26, so if his fitness is aceptable, and it appears to be, then I think there's enough reasons to try it.