Most people will agree that online play is not neccessary, but it's a very good luxury. When you think of the other two systems in Playstation 3 and XBOX 360, both have well established online. Well, where's the Wii's? Yes, we have received games that have mediocre at best Wifi in PBR, Strikers, Medal of Honor, and others. The games can get a good "quick fix," but leaving us empty after a few weeks.
The other systems main online games in Halo 3, Gears of War, Call of Duty 4, and Warhawk are STILL played at a constant rate. I personally own an XBOX 360, and there's still at least 5 people on my friends list playing Gears, and that is a year + game. The Wii is triumphant. However, triumphant in dishing out single player games. If you think about it, Metroid Prime 3 Corruption could of been AMAZING with online play. Different colors/ranked Samuses and the new suit upgrades come after leveling up. I agree, games like Super Mario Galaxy, Zack and Wiki, and Zelda don't need online, but Mario Party, Metroid, and WarioWare could of deserved it.
The Wii has too many games where you play and it's done. I think the only exceptions are Galaxy and Guitar Hero. I honestly, do not want to replay Zelda, Metroid, or even Resident Evil any time soon. Oh yeah, Brawl may change, but if we're spending $50.00 a game, we should be able to get the full value of it with very good online play. Now, I don't rely on my Wii for online play (hence owning a 360), but it could of really justified multiple plays when completing the single player. It's just my .02 cents and I am sure I am beating a dead horse, but I just really want the Wii to succeed in online play.