Most people have UPNP enabled anyways and some applications use it. In my opinion there is no reason not to support it. If it is enabled on the router there is no reason why you shouldn't use it and if it is not, you can still fall back to manual port forwarding.
Games with only dedicated servers don't really need it, but and it is only something the server has to deal with, but if you allow the player to host a server from within the game, just support UPNP, it's great because it just works. And even though there are some security concerns with it, just using it if it is enabled, doesn't change anything about it.