I was under the impression that the carriers and manufacturers modified their phones so they could differentiate themselves. There's the revenue opportunities, sure, but they have to do something to stand out.
Now here's the crazy part: no one is selling a device with stock Android. If your phone was straight Android 2.2, it wouldn't look like any other phone on the market. The minute Motorola or LG realize this, they get 100% of the nerd phone market.