My understanding from reading quite a bit of lore on dragons in TES for quite some time is that dragons are one of the Akavir races and suspected to
possibly be extinct. It seems dragons are shapeshifters however and probably very good at remaining undetected. The lore is clear that their extinction is not confirmed. I find it consistent with lore to suggest that there just may be a few dragons around.
Being able to summon your own to ride however is, IMHO, quite a stretch although great fun I'm sure. Your call on how lore friendly it is. My point is that the lore is clear (to me at leasts) that dragons just
might still exist.
You might want to do some reading on it for yourself and see what you think. Start here: http://www.uesp.net/wiki/Lore:Dragons