My understanding is that Coulson's death was intended to be "for real" in The Avengers, and that his resurrection didn't come about until they were developing the TV show. If Marvel was willing to kill him, why wouldn't they be willing to leave him a vegetable?
It's Marvel. The only person who stays dead is Uncle Ben. Heck, the comics' Peter Parker just spent the past two years dead (for tax purposes) and was recently brought back in time for the new Spidey movie. And now they've announced an event called
The Death of Wolverine (they removed his healing factor a while back to set it up), but it's a cinch it won't be any more permanent than his previous three or four deaths.