I dunno, I think that being a Templar (or ex-Templar) when there is the giant back story of the mage rebellion going on... I think he would have a lot of interesting things to say.
I was thinking more of a clear character arc. Usually, characters unveil their background over the course of the game. At the same time, they get their own significant plot that you play a part in. We already know all of Cullen's background already (the interesting bits anyway). That brings up a smaller issue: how will first-time players get to know about him without boring the dedicated fans?
He could still have a character arc, I suppose, but it just feels to me like the big events in his life are over. He was broken at Kinloch Hold and got remade hardier and more level-headed as Knight Captain at Kirkwall. That's a pretty solid and complete storyline for a side character. If it gets extended, it might feel a bit like the plot lines of some of the major characters in WoW: very much a series of major life-chaning events one after the other.
That's pretty much the only reason I'm not keen to see Cullen as a companion. Much more fun to have a brand new character to fangirl over.
Edit. I thought that hindsight would take a bit longer to kick in than 30 seconds. I'm sure the writers will work wonders.