GM likes to please people. Problem is, it's impossible to please everybody with the same car. I guess they could sell all their cars under the same brand, but each of them have become, over the years, viewed as "target buyer specific".
They're trying to change this, but to me, the GM brands go (or went) as follow:
GMC: Trucks
Chevy: General Consumer cars (mostly)
Ponctiac: General Consumer cars (mostly)
Buick: Cars for old people
Oldsmobile (just for the purpose of argument): Cars for old people
Cadillac: Cars for old people that are trying to spend their heritage before it's too late
Saab: Buy European!
Opel: Pontiacs!
Holden: Pontiacs!
Saturn: Cars made out of plastic that would have dominated the market, had they had any other arguments than the plastic body
The way I feel about these is probably not the way everybody feels about them, but I am quite sure that a lot of people associate each brands to the people who buy them, like I do. That's a major problem for GM when they're trying to drastically change the way people feel about their cars. Technically, GM would only need two brands, maybe even one. You could argue that having the trucks and the cars as separated entities is reasonable, but beyond that, it's pretty much useless to divide. You don't need a brand for luxury cars, it's just to fool Cadillac consumers into the false feeling that they're buying something else, better than a Chevy. In most cases, it's just a re-branded car. And it's just not the Americans that do this.
Also consider that the American consumer is very attached to those names. People wouldn't let go off Cadillac easy. A lot of people are already sad to see Pontiac get off the boat.
I think Saab should and could have been saved. To me, Saab cars have always been very good looking, but awful to drive (sloppy) and, mostly because of this, not good value.