In that case, if you're trying to distribute through a portal that requires mouse control, then yes, mouse control is important. Maybe not to playing the game, but to getting the game accepted on that portal. Who knows WHY customers of that portal prefer mouse control? It's
probably because all the other games they play, from the same portal, use mouse control - even though they're different types of games. They may just be used to playing games with the mouse and don't like having to use the keyboard instead (as simple as a behaviour/routine they are uncomfortable about changing).
If the question is "do you NEED mouse support?" then the answer would be a definite NO. It's already proven that you can play the game without the mouse, as there IS no mouse functionality built in by default (unless you've added something to the game that can ONLY be done with the mouse).
But if the question is "should you PROVIDE mouse support?" then it comes back to the audience, and whether they will find it more or less difficult to play with the keyboard or the mouse. And to the game itself, and whether there are any parts that will be more or less difficult to play with the keyboard or the mouse.
And that question can only be answered on a per-game basis. There is no single answer that covers all games.
