It would have been ''a lot of fun'' to have Sinise as McCoy, and if this picture had been set later in the ''timeline'' (whatever it might be now), he would have brought such a presence.
However, let's not prejudge Urban's take, or J.J.'s for that matter, sight unseen.
We had a lot of fun on the Star Trek New Voyages forums when news of the movie broke a couple years back. We did a lot of ''casting'' among which Sinise was put forward as McCoy. Some various ''high concept'' ideas were thrown around including Angus Macfadyen (best known as Robert the Bruce in Braveheart) as Scotty, Jessica Biel as ''Number One'' if she were an element. Interestingly enough, if Sigourney Weaver were much younger (or the movie took place much later), she would have been the perfect Number One (thinking Alien(s) not Galaxy Quest). Liv Tyler as Number One was another fun idea. If Kirk's first captain, Charlie Garrovick were to make an appearance; Ed Harris. And, if this movie had been later in the timeline, why not reunite Sinise with Hanks, with Hanks playing Spock?