That about summoning them... I would suppose you actuallty force them to help you.
And becoming into a dragon has nothing to do with what Martin did. He just acted as a receptacule for Akatosh. Becoming into a dragon would be, in my opinion, a stinky overpower idea.
one can dream, being a dragon or riding one is one of my favorites things i like to see in fantasy games. there could be even some practical uses for a "dragon form" like reaching an otherwise impossible platform in the highest mountains to find a few unique dungeons or having dragon on dragon air fights.
not all dragons have to be hostile to the dragonborn.
what if you could befriend one? being intelligent creatures themselves i'd imagine friendship/respect for the dragonborn isn't impossible under the right circumstances, like freeing one of them who has been shackled or healing one who is on the brink of death.
we'll just have to wait and see.