If I build protection into a game which relies on a server and a database to generate keys and keep track of legal versions, then that's an expense that I'd have to pay and as soon as it becomes unprofitable to keep that server running, it would be shut down. If this server authenticates each time the game starts, that's it.. game's dead. No more.. finito.. unless, someone hacks it, removes the copy protection and releases a version on the net. Or I release a version without protection.
Jason, one check per install/update is adaquate. That means they can install it and then use it while offline. And if you are going to remove the server email all your registered users and give them a full copy of the game minus the protection as a reward for sticking with you. You'd keep me plenty happy this way. Or if once per update/install isn't enough for you, just make it check weekly or monthly. The always at startup check isn't really required.