It isn't fan demand
In regards to Cullen...
IIRC, he was created as a one-off character. Sheryl never expected people to grow attached to him. There were originally no plans to bring him into DA 2, which is why he goes insane and becomes a mass murderer in one of the epilogues.
The fan reaction to him was so positive that they put him into DA 2.
When they started DA I, Cullen was a good fit for one of the advisers. He was a Templar, in Kirkwall, could survive until the end, and because we didn't know much about him, there was lots of room for growth. After all, Cassandra brings together the advisers and the last time we see her, she's in Kirkwall.
I am sure that his popularity made him a natural selection for an LI. If people hadn't liked him, I doubt he'd have made his way into DA 2.
This is not saying that I consider him fan service, at least not any more than Varric, Leliana, or Cassandra.