There's probably more to it. I'd be surprised if it behaved like a normal summon. Perhaps there's a chance of it failing, like if a dragon isn't nearby it won't work. Also we don't know if it's going to be a dragon doing a fly by and laying down a few fireballs or if it'll land and bite and tear enemies. Also it's likely they'll only stick around for like 10 seconds.
Yea there is just too many possibilities of what it means to summon your own dragon with the shout. Having the dragon we saw in the gameplay trailer with full AI (flying up in the air, melee attacks, fire balls, aerial based fire attacks) at your side for x amount of time to fight for you is pretty cool but there will for sure be restrictions if that's the case I have no dout. So yea realistically we will probably get some sort of dubbed down dragon with its full combat AI & abilities or just a simple aerial fly over attack.
But as johner210 said, a dragon coming to you only if there is one nearby would be
VERY AWESOME I think and the amount of time you are in control of the dragon could vary as well. That would balance out a lot of things like attacking & devastating towns, summoning them into caves and inclosed area's etc.