Both MLB 2K7 and MLB 09 had to, and in the case of 09, if you did the update and tried your current franchise, it would lock it up. Also, some games might not have
required a restart to play, but you wouldn't get the update in an already started franchise (has happened with Madden a few times, NBA 2K, etc).
It doesn't happen often, but it's a legit question. It only takes one time getting burned to make it an issue for people...no one wants to invest time into a franchise and find out they'll lose it.