Mafia 2 will be out for PC and will beat all GTA games easily ;\)

But seriously:
Wii has a few advantages. They have funny casual games, family games, (plus hardcore games) and the price is much lower compared to PS3 and XBox360.

That is a reason for many beginners, young and older people to give it a try.
This approach was a good one and prooved to be successful for Nintendo.

By the way: there are powerful engines now available to allow beautiful graphics on Wii as you can see here (Quantum engine from High Voltage software):
http://youtube.com/watch?v=hkNguFQBAqQ

My personal point of view:
I ignore all consoles because I work daily on PC, modelling, painting textures, programming software. If I take some time to relax then I can play fantastic epic games like Witcher, Mafia, QuakeWars without the need of a console. But if I want to play with my kids anytime in the future I would instantly get something like the Wii instead of PS3, just for the sake of fun and family play.

Models, Textures and Games from Dexsoft