It's both, actually. It's not my only reason, though.
I am sure if Shatner did get a shot at reprising the role of Kirk, he would ham it up, and he would likely be a caricature of Kirk, resembling more of a Denny Crane than classic Kirk.
But yeah, I have a personal problem with Shatner too. He's a great actor, but my personal opinion of him is really low. Your mileage may vary, but I have tried ignoring it for a long time, and like you I took the attitude of who cares what actors do. However, there is just a long track record of him being nasty to fans and coworkers, and it just finally got to a point for me to not ignore any longer.
