Then my guess would be that the dragons that aren't hostile are essential to the main story and cannot be found outside of it.
Edit: ninja'd :toughninja:
That's actually a pretty good point and brings up an odd line of thinking from me:
What if the shout that summons this supposed dragon ally, is obtained via the story from the very dragon you summon?
Instead of being some temporary or limited minion with no real characterization, the summoned dragon ends up being an actual story character of some sort? The question then would be the means of obtaining it.
Does the dragon come of its own free will via some arrangement, or is it enslaved via your shout? If the latter, picture said dragon, possibly cursing you in its dragon tongue every time you summon it.