I own a Wii because it has games that will NEVER EVER EVER come out on the PC. Period. If you own any other console and a gaming computer you are an idiot.
You can say that for most consoles.
There are game series that are bound to certain consoles (e.g. Metal Gear (except for I and II on NES), Final Fantasy (except for I-VI on NES/SNES) and Gran Turismo for PS consoles, (most) Halo titles for XBox/Xbox360 (except for I and II on PC) and Mario games for Nintendo consoles (except for Mario is Missing and Mario's Time Machine on PC).
So if you like a certain series and want to play it, you're bound to buy the corresponding console - which isn't only the Wii.
Also, Wii games are usually cheaper ($10-$15) than any other console.
That might be because they look (in terms of graphics) inferior to other consoles - so people would basically be thinking: "Why buy a certain game on the Wii (resp. buying a Wii for a certain game) if it was a game, that's also on another console looking way better there?"
An example would be Ghostbusters which came out for DS, PS2 and Wii coded by another developer team than for Xbox360 and PS3, which points towards the Wii's limitations (on top of that the Wii/PS2 Ghostbuster game has some bad gameplay elements to it, that make it less enjoyable compared to the other versions by the other developer team). So if we're talking "NextGen" the Wii isn't all that NextGen at all. It has it's own special input devices, though.
Still it feels to me as if they put all their money in that in the beginning and still the Wiimote without MotionPlus is a pain in the ass. I'd rather use a classic controller most of the time.
This is where the Wii _STILL_ shines. Lemme list the games I have played with my friends in crazy 4-person mayhem: Mario Kart Wii, Super Smash Brothers Brawl, New Super Mario Brothers, Wii Sports.
None of those games are available anywhere else and they all are amazing. Seriously, quit hating on the graphics that does not, and will NEVER, make a game GREAT.
I have to agree with you about the graphics not being able to turn a shitty game great.
Yet 3/4 of the games you mentioned are from the Mario series that started way back on another console (SNES, N64, NES respectively), proving my first statement. As for Mario Kart and Smash Brothers (well that's just my opinion) I prefer the older versions because of some aspects. Nevertheless, there's no doubt that also the Wii has good games, e.g. Super Mario Galaxy 2 (despite outdated water effects and some aliasing effects) is just fun to play and makes perfect use of the Wiimote and Nunchuk.