He doesn't necessarily have to stay away forever. He's the Master, so--like the Cybermen--he can come back eventually. His first appearance in "Utophia" was a nice twist, even for new viewers like myself who had no idea who the hell he was supposed to be. His reappearance in "The End of Time," (also known as The End of Ten), however, already took some serious suspension of belief, and he'd already been gone for a season and a half. He was a Time Lord that was somehow coming back to life. It would have been one thing if they'd had him regenerate somehow and come back as a new actor. As it was, his triumphant return (and bleached blond hair) was pretty implausible, even for Doctor Who. Although he got one of my favorite lines from the "The End of Time" two-parter, he would have benefited from a longer break and the surprise of a new actor. Moffat has so far seemed to prefer to keep away from the Master, so it's entirely possible that the character finally did die off with Davies' departure.