Wii U version will allow the game to be played in a pragmatic way during actual Smash Scene tourny events without the need of extra equipment. I know that Touhou has been a big part of the appeal for a lot of people here. But there are also quite a lot of fans of the game for it's competitive smash design.
Steam: As I've said before, many people, even including people who aren't saucers, have expressed that they either want this on steam or would be more interested if it will be on steam.
It's that simple.
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If there needs to be more reason, I could make it to where the Steam/Wii U version also includes some additional characters that are simply apart from touhou.
You don't see what's wrong with that?
There's no point in having the game on steam if it's for free on PC in it's Touhou form. People who are only interested because it's on Steam is just plain silly when they can get it in a simple location for FREE
You're simply doing extra work at that point.
YES you NEED characters who are apart from Touhou or at least only inspired ones featured ingame for the other versions.
It is okay to have inspired characters similar to how sonic the fighters had Honey the Cat where she wasn't a carbon copy of the actual Honey and was rather a cute reference.
But re-skins is effectively the same as putting a H-game with censors on steam when you're selling it/providing it without censors elsewhere, it's really not worth it.
As for tournys as far as I can tell you had that Touhoucon thing done without much issues and it wasn't even close to how it is now, though if a game is destined for competitiveness it'd be done regardless of platform