it's probably more like summoning than actually calling. and besides, it's not like you could call the 60+ dragons in Skyrim to aid you. you can only call one (unless there's a shout that's "summon dragon army".
I want to know what type of situation your in if you need to call a dragon to save your ass.
Maybe if one is attacking you, but what does that mean for the story? Some rogue dragon just feels like helping you kill that mudcrab?