I could be wrong but I suspect many publishers deal with studios like Platnium Games/Clover Studios based on the long term potential, not necessarily on the sales potential of the game at hand ... Essentially, if you have a studio who reliably produces critically acclaimed unconventional games for a long enough period of time you will (eventually) see a breakout success that makes the expense worthwhile; at the same time, if the studio builds a large dedicated following through this process then their upcomming games (regardless of if it is a new IP or not) will likely sell very well.
The Wii works well for this at the moment because development costs are relatively low and (because pushing the most technically advanced graphics does not really impress that many people) developers are focused on more artistic graphical styles which can result in far lower development costs.
With that said, people who have actually played MadWorld would probably agree with me that there are signs that it was developed on a very small budget ... There really aren't that many character models, the environments are pretty small, and there is a lot of repetition of content; while watching the games ending credits the most shocking thing is how few people were working on the game, and it seemed like the original Super Monkey Ball on the Gamecube had a far larger development staff.
