It's a gamble but it's got it's postives too, you end up with a few flops then it doesn't cost you as much as having the likes of Doyle, Johnson and O'Hara stuck around for 2 years or more when they've nothing to contribute.
A successful permanent signing would obviously be the best option but I'd rather than a better player on loan, if we're talking season long or at least window to window, than end up with a substandard player just because they're locked down for longer.
I think it'd be silly to turn down the opportunity to sign a better player, especially if they would improve a particularly weak position, just on principle because you can't guarantee you'll get them permanently