Its actually an increasingly popular trend, for a number of reasons I've heard, including:
A - Ease of transition to PC from Console gamers (for instance, I've been a console gamer my entire life and I don't always find keyboard and mouse the easiest to control for some games)
B - Can be more comfortable for some people to play, especially those with Carpal Tunnel Syndrome.
C - The introduction of Steam's new controller is leading towards much more common support (and use) of Gamepad support for popular PC games. There will be people who expect this moving forward, and in increasing numbers.
Its still a rather niche group at this point IMO, but I would wager that in the next few years, it will become much more common.