If you've got a taste for plain old vanilla Android Froyo or Gingerbread, finding a smartphone without all of a carrier's and manufacturer's extra toppings is harder than you might imagine.
Custom interfaces that sit on top of the Android foundation cost phone makers (and sometimes carriers, who can also weigh in) time and money to develop and deploy, whereas Google gives its basic operating system away for free. An extra interface can also cause extensive headaches when it comes time to graduate a handset to the next dessert-y Android OS.
In other words, leaving a phone in its pristine Android state would be simpler and cheaper for most smartphone makers. Opting for stock Android can also help manufacturers hit a lower price point on a phone, and the simple act of leaving well enough alone could lead to less grousing from customers, who are easily and justifiably angered when an extra interface layer causes delayed or broken Android updates.
On the flip side, it's also known that custom interfaces like the Droid UI, Samsung TouchWiz, and HTC Sense are instrumental in helping phone makers differentiate themselves in terms of extra features and a classy, edgy, or professional look and feel. To preserve a brand feel and identity, phone makers overwhelmingly opt for a custom UI, which is why a simpler Android experience is less appealing for them than it might seem.
All right, fine. Then smaller manufacturers should tend toward the skinless OS every time, or maybe prepaid or regional carriers are predominantly offering nothing-but-stock-Android smartphones.
That all makes sense in theory, but in reality it's not that simple. Indeed, while a phone like the Samsung Galaxy Prevail for prepaid carrier Boost Mobile fits this theory by forgoing Samsung's extra TouchWiz interface, there are plenty of smaller manufacturers that go ahead and add their own twists to Android (like Casio with its G'zOne Commando), and there are plenty of very basic smartphones that carry their mighty manufacturer's interface paradigm (like LG's Optimus series).
By the same token, you find high-end phones like the Motorola Triumph (on the prepaid Virgin Mobile network) and LG's T-Mobile G2x, that are simply dressed in Android. (We rounded up some of the most recent specimens in this gallery.)
The rule, as far as we can tell, is that there is no rule, and this seems fitting. Were phone makers to mostly supply their most basic phones with unskinned Android, customers with shallower pockets who prefer the visually different experience of a custom UI would lose out. So would Android as a whole, its original effort becoming relegated to the "budget" sphere. Likewise, phone owners seeking a premium experience on any carrier deserve to choose among phones with a range of interfaces, stock Android among them.
As personal fans of Google's simple look and feel, we can only offer this advice to carriers and manufacturers: keep 'em coming.