Are there enough solid Wii fighters to justify it?

no

Tatsunoku Vs Capcom can be played with a sideways wiimote. I can't think of any other fighting games. Don't bother.

You could possibly buy it, mod it with better buttons, PCBs, multi-console, etc. To use the stick ONLY for the Wii...no.

" @OllyOxenFree: Hmmm, do you know if the Madcatz SFIV sticks are multi-platform usable? "

" @blueduck: Weird. Aren't they USB? "

I think I might pull the trigger on this. Apparently it's easy to connect it to a PC through Bluetooth and you can't go wrong with $30 fight stick, especially considering it'd be my first one.

As long as there's no lag with the bluetooth, then why not? An inexpensive, wireless fight stick is a handy thing to have even if you're not really into fighting games. I've used my Hori EX2 with Super Meat Boy than with all the fighting games I've played with it combined.

The Wii has some great stuff like the King Of Fighters and Samurai Shodown collections as well as some Guilty Gear and the aforementioned Capcom vs Tatsonoku 
All games which you can find dirt cheap now. 
And they're still cranking out the occasional gem on Virtual Console. Who knows, maybe there will even come a good fighter out on WiiWare... 
Personally I'd go for it.