I'm really happy to see Cullen in a prominent role in DAI, and I think that his personal history makes him an excellent choice. And I'm not even a Cullenite, just really intrigued by his character arc so far.
I know this isn't the sole reason for some players disliking Cullen, but it's getting tiring to see guys harping about his inclusion being fanservice, etc. Cullen is interesting and had a complex multi-game character arc on par with characters like Isabela, Anders and Leliana, that's why he's being included. There's also the added value of him having personally experienced the worst of both sides of the mages/templar conflict that's currently boiling in Thedas.
As people have pointed out in several threads, the timeline shows us that the templar war has been going on for 3-4 years at the time of the peace talks. During this time, Cullen has presumably been trying to keep order in the melting pot that is Kirkwall after the destruction of the Chantry and the following circle rebellion. Adding what happened in Asunder a couple of years later into the mix, and it's really not that hard to see why Cullen would be an interesting character to explore in DAI. What other templar have we met previously who could bring the same complex narrative to the mix? Cassandra's choice to bring him into the Inquisition seems more than reasonable to me.
Having a character whose opinons or previous actions you abhor in the game is nothing new. I absolutely hated what Loghain did in DAO, especially towards the elves and how he sacrificed King Cailan, but I still enjoyed making him a grey warden and see him work to redeem his previous actions. It made him more interesting and complex as a character and I was pleasantly surprised to see his cameo in Awakening. Reading The Stolen Throne and The Calling later made me sympathetic to his paranoia towards Orlais, and eventually I even sort of came to understand his disappointment in Cailan, the son of Maric whom he was so devoted to.
So if a player like me can reverse my opinion of Loghain, is it really so impossible for others to reverse or at least enjoy the appearance of Cullen in DAI? He might even end up as your new favourite character. You can't know that before you play the actual game.