Well, firstly, I refuse to play roms of games from any current game systems, whatever the case. The only way I ever play gamecube or wii games is on the actual system (besides, there's no real way to play games from those systems on your computer yet, but if there was, I wouldn't do it.)
Now, on the other hand, I see nothing wrong with playing games that are not readily available. A couple years ago, I constantly played Smash Bros and Mario Kart for the N64 on an emulator because buying a real system would be much harder (yard sales, ebay auctions, and all second hand ways were the only way to go, none of which are very reliable). Now days, with the Virtual Console, actual versions of many games are available for download from Nintendo, so I don't play emulators actively anymore, unless its a game thats not on the Virtual Console yet.
Basically, I'm against just playing games on an emulator if you don't want to pay a few bucks to Nintendo and play them on the Wii. Other cases are fine with me, though.