I'm a diehard GM fan, but you can definitely question some (ok, a lot...haha) of their moves.
However, look at how many vehicles they are building between each of their brands. Unlike Nissan who seems to only have like three engines between two brands (2.4 I4, 3.5 V6, 4.x V8), I don't think GM could pull that off. I just don't see that working for a manufacturer that has so many different types of cars/brands.
If you think about it, GM doesn't really have all THAT MANY different types of engines. The 2.8, 3.1, 3.3 (i think), 3.4, & one of the 3.5 V6's were based off the same 60* pushrod engine.
GM currently has two different 3.5L V6's. The first one (G6, Malibu - 200hp) is the "older" V6 that is basically just a bored out 3.4L V6. The "new" 3.5L V6 is a new design (2006 Impala - 211hp) based off the new 3.9L V6. Both are 60*. The old 3.8L V6 was a 90*
So yeah, GM is somewhat confusing when it comes to the V6 lineup, but if you step back and look at it, they just have tweaked a lot of things that make it look like they have tons of different engines.
*For those that are familiar with GM and their engines, feel free to correct me if I am wrong in anything I wrote.