They still had to pay Shatner's fee, and at that point he didn't get paid by the page. If they had to fork out the cash to get him, it made sense to use him as much as possible.
True, that.
Twenty years on, I'm still surprised that Shatner agreed to do Generations. On the DVD commentary, Moore and Braga talk about how Shatner pointed out that Kirk isn't necessary to the film's story and how they never solved the problem.