- Chevy brands now market mostly to basic- and budget-minded people
- Pontiac goes for the adrenaline-rush crowd (think Trans-Am)
- Buick aims for the middle, between sporty and luxury, but upgrading its image towards the mid- and high-end isn't impossible for this lineage.
- Cadillac, in my mind, now represents overly-affluent, "fogies". Over-sized boats, in other words.
With all due respect, I think you're view of what GM makes today is about 10 years out of date.
Pontiac was marketed as the "excitement" division. But what do they make _today_ that is exciting? The G8 and the Solstice. The G8 is very well regarded, the Solstice has done ok, but neither are selling very well. There hasn't been a Firebird/Trans Am for 7 years. So if you want to keep Pontiac for the adrenaline-rush crowd, they're going to have to create a new line of products that are actually interesting. Selling clones of the Chevy Aveo, Chevy Cobalt, Toyota Matrix, and Chevy Equinox does not a division of excitement make.
Cadillac is probably GM's big success story of the past 5 years, turning the whole line around into a bunch of generally well regarded vehicles sold worldwide, especially the CTS, which isn't anything close to an oversized boat.
Buick sells more vehicles worldwide than Pontiac. They're not terribly exciting, but worldwide brand recognition goes a long way. Pontiac is only sold in North America.
What I would've done, instead of dropping Pontiac, was slim down the crossovers (Buick cars that share similarities with Chevy cars, for example), kill the Cadillac brand, spin off Saturn (like most Saturn fans wanted for a long time), sell off Hummer to someone else (hopefully killing those godawful H2 and H3 SUVs, and bringing back the "true" H1), and negotiate on more level-handed terms with Sweden about letting go of Saab (or is it Norway? I forgot).
Pontiac has a lot more rebadging going on than Buick. Buick only sells three models in the US market currently. The Buick Enclave shares platforms with the Chevy Traverse, but is more luxurious and styled completely differently. The Buick Lucerne is closely related to the Cadillac DTS and built on the same line. The 2010 Buick LaCrosse is nearly a joint China-US product built on a platform shared with a number of GM vehicles, including the Malibu.
I wish the future Saturn the best of luck. They're going to need products from some manufacturer as they're going to be a brand that is nothing more than a dealer network.
General Motors owns the "Hummer" brand, and that's what's for sale. The H2 and H3 are based on GM platforms and parts. The H1 was the civilian version of the military HMMWV. The H1/HMMWV are all built by AM General. AM General quit making civilian versions as they were only selling about 100 a year and needed the capacity for military production. Any sale of "Hummer" is little more than the brand name. The classic Humvee stays with AM General.
Sadly there's little to be made from Saab.
It's unfortunate that GM has ended up in this current state of affairs, but they have very little options left. Here's what they're probably going to be left with at the end of the calendar year:
Opel/Vauxhall in Europe/UK, probably selling a large stake of it to generate funds.
Holden in Australia.
Chevrolet worldwide
Buick worldwide
Cadillac worldwide
GMC in North America. I suspect GMC branded vehicles' days are numbered as well but has a few years left in it.
GM-Daewoo in Asia (branded as Chevrolets in many markets)
Sold or killed will be Hummer, Saab, Pontiac, Saturn.