Same with Sebastian. At least, I think having him around and his scenes with Elthina made the uh... climax more shocking.
That said, I don't think I would like Cole. If he is a mage... I don't want another blood mage. I don't want another abomination/mage. If they DO have another "spirit sensitive" mage like Wynne or Anders, I think Solas would fill that role. I am hoping that Cole's spot from the survery went the way of the dodo when they came up with Vivienne, Solas, and decided to bring back Varric. Maybe Varric took his spot if he was going to be a rogue.
I'm pretty sure that Solas, Vivienne and Varric were all *internally* designed and confirmed as companions months if not years before the survey was even published. I don't think it's accurate that the roster of survey companions was everyone that they had in mind, and everybody else was created later. Given that they were aiming to release in late 2013, I imagine that the companion list was well and truly finished by August 2012, if not significantly earlier. There simply isn't time to cut and redesign people like that, not when so many other departments (art, cinematics, the other writers) depend on things being locked down as early as possible.
In fact, David even said just a few days ago that Maevaris Tilani was created for the comics *too late* to make it into DA Inquisition in any meaningful form, and her first appearance in the comic was the same month (August 2012) as the survey came out. She would've had to be designed months before that, given how long those comics take to produce.
When you think of the amount of work that goes into concepting, designing and implementing companions into the game, I just can't imagine that they'd cut and change people as easily as some fans think. By the time that survey leaked, I'd expect it was far too late to change the companion roster as drastically as cutting three people or whatever - and that aside, I'm pretty sure Varric, Vivienne and Solas existed anyway (just not in the survey).