While I agree that over-reliance on franchises can be detrimental to the overall market and that good franchises are too often used to move mediocre product, I think there are some benefits to franchising product (games or otherwise) that are being overlooked.
The main benefit to customers is that a franchise sets expectations. You put in a Mario title and you can expect 3D platforming of a certain quality. In fact, the biggest complaints about franchise titles come from instances in which the new title doesn't meet the expectations associated with that franchise. The classic case of this was Super Mario Bros. 2, which was superior in gameplay to its predecessor, but met with resistance because it felt like a completely different game because, well, it was. There are plenty of more recent examples, like Super Mario Sunshine that had more emphasis on "graffiti" than platformimg or The Legend of Zelda: The Wind Waker, which diverged aesthetically from the realism of the N64 titles.
In other situations, the franchise continues because developers want a version of the popular game available on current generation hardware. Consider MarioKart or Super Smash Bros., both of which have released exactly one iteration of their game per console. And his is perfectly reasonable, because once the new generation of console comes out, previous comers are nullified (with the exception of the PS2, which stubbornly refuses to die). Once the Wii came out, nobody stocked new GameCube titles, and since those games have huge fanbases, it's reasonable to expand the IP.
There are benefits to the developer as well. Continuing a franchise means less character design and a way to integrate ideas and improvements that didn't make it into previous installments. Look at both the Half-Life and Left 4 Dead franchises, in which the subsequent games add new dynamics and subtle improvements while building on the design (and in these cases the story) of the games that came before. This can also cut down on development time--again, look at L4D2, which is a better game than L4D but took a third as long to make.
So it should come as no surprise that Super Smash Bros. Brawl would come out for the Wii shortly after it's released. And if you're familiar with the SSM games, then you immediately know what to expect and whether or not you'll like it. And while you're free to lament the lack of original IP's out there, shitting on franchise games just for being franchise games seems a bit petty to me.