IMHO, the biggest problem with the TNG movies wasn't the fact that they were written by television writers. It was the fact that the writers weren't allowed to just write.
For Generations, Moore and Braga were given a laundry list of requirements from the studio and then had to construct a story that met them. For the remaining films, Stewart and Spiner kept mucking about with the story. Stewart, in particular, seemed hellbent on re-imagining the character of Picard as something entirely different. It comes back to my basic premise that you should never, ever, under any circumstances, for any reason what-so-ever, let actors who are acting in a project also be involved in the writing in any way.