I know where @Rick is coming from and I do partly agree, at his age he ideally wants to be first choice somewhere.
What we don't know is what offers he actually has/had. Imagine joining QPR for a season, 46 games and collect £15-20k, or back up at Spurs, play a few games and get £40k-50k? You could see why he'd choose the latter.
Also on the flip side of that as @Stansaid, Spurs have so many games with Europa League and domestic cups, you'd think he would get a look in somewhere and whose to say it won't be at the expense of Gazzaniga? Did he really take his chance when Lloris was out for a while mid season? I'm not too sure Jose trusts him. Maybe he wants a bit more experience there.
If for whatever reason Lloris got injured again, Jose has always preferred experience over youth, I could see him playing Hart over Gazzaniga.
And for all we know, Jose may have brought him for reasons that aren't obvious. Vastly experienced, vocal in the dressing room, someone who can pass on knowledge and insight to the younger players, maybe just as a personality? I'm sure Mourinho alluded to his team as a good guys but they need to be more nasty or ruthless? Well, maybe Joe Hart has that personality, we know he's always been very confident in his ability.
Either way, I don't think he has been brought in completely on a whim. It's been well thought out one way or another.