Well, you have to look at why they customize the OS in the first place. And it's not because "stock" Android is ugly. It all boils down to one word: distinctiveness. Consumers want the "best" (read "most distinctive") devices, with the most flash, flare, and eye candy. Mobile phone vendors can really only add distinctiveness through more/better hardware (like an amazing display, faster processor, front-facing camera, a camera with a flash, or a mood rock skin), or though the software that ships with the device. Given that all manufacturers have access to the same OS (Android) from the same vendor (Google) at the same price (free), this isn't a source of distinctiveness until it's modified. Adding all of the best hardware to a single phone may produce a killer device, but it's price may be considered grounds for murder. On the other hand, sprinkling a custom UI on top of Android may turn mediocre hardware into something to die for at little or no cost to he manufacturer.
There are a few other (potentially minor) reasons that it makes business sense for the practice of "frosting" Android to continue:
- Economies of scale. Once a company like HTC has made the investment into Sense UI, the cost to add that to any phone in their lineup is minimal.
- Adding value. Once a critical mass of consumer deems that Sense UI (or Motoblur) is "cool", this becomes a source of value to the manufacturer. They can charge much more for the phone by "adding value" to Android.
- Reducing dependence on Android. As good a deal as Android is to handset manufacturers, it only makes sense to preserve future revenue options. The first time we saw Sense UI, it wasn't running on Android -- it was running on Windows Mobile. And, if Android dies tomorrow (or Google decides to charge confiscatory licensing fees for Gingerbread), HTC could spend the time to port Sense to another OS (or write their own) to preserve their revenues.
- The UI is part of the experience that the phone manufacturers offer. It (can) provide some consistency between different devices (even if those devices run different versions of Android, or different Operating Systems entirely).
- Sunk Costs. HTC and Motorola already have significant investment in their customizations. They're not going to walk away from those investments without milking it for all its worth.
Overall, as much as I'd like to see these custom interfaces go by the wayside, I don't see it happening anytime soon. The only thing that will get a significant number of devices (read "more than one") that has great hardware under the hood, and is updated with the latest Android release as soon as it's available is for consumers to band together and demand it. In short, consumers need to demand another Nexus One.
Update: If you agree that we need more Nexus One-like devices, there is a petition here where you can let device manufacturers know that there is a demand for Android devices with minimal customization. Perhaps a show of numbers may tip the scales in our favor.