If VC was had online co-op/versus, I'd consider it a killer app.
What they need to do is get more rare/import games on there (Dracula X DAMMIT, better NeoGeo goodies).
Alien Solider, Sin & P, Lost Levels are good starts, but we need more. I don't think most Wii owners probably are into VC, outside of buying a Mario/Zelda or two.