Snes9xGx is a good enough emulator, I don't see why Snes emulation would need to be implemented into Mednafen.
Same could be said for Master System and Genesis... still it's good to have alternatives.
Also I guess having everything in the same app might be nice for people who just want to play various games briefly.
Anyway, Mednafen SNES emulation is based on BSNES, which contrary to other emulators, has never been ported on Wii.
This only makes a good reason in my opinion, even just to test how fast (or more likely how slow) it runs.
That's probably why razored probably decided it was unnecessary efforts to implement it at the moment.
Similarly, Mednafen also supports PSX, might be interesting to see how it compares to WiiSX