2008 GM Vehicle Sales
Chevrolet 1,801,131
GMC 376,996
Pontiac 267,348
Saturn 188,004
Cadillac 161,159
Buick 137,197
Hummer 27,485
Saab 21,368
Total 2,980,68
I hate to say this, but GM doesn't need Pontiac. The G8 and Solstice, which could easily be rebadged. GM's only brands that have significantly different models from Chevy are Hummer (no need to explain why this needs to go), Saab (sales unfortunately don't justify keeping), and Cadillac.
Looking at the sales numbers above, all of their other brands put together can't add up to Chevy's sales. As a high end luxury brand, Cadillac doesn't need high sales volume to make money. However, Cadillac's the only GM brand who's name has enough cachet to mark up its cars enough to be profitable without high volume.
The multiple brand paradigm worked in the 50's, but times have changed. IMO, GM really only needs Chevy and Cadillac.