I worked at one of the big 3 TC hardware companies and we used TCs for everything. All the developers used them to connect to their own VMs, and that is where we did all our development. It worked very well. We even had TC notebooks, which I saw being used "in the wild" by a librarian at our county law library. Our CEO's theory was that if we were going to sell our product, we needed to use our product.
That being said, you're absolutely right that they are not aimed at developers. They're aimed at any company that wants a simple, secure, easy to manage, and secure (yes I wrote secure twice) device that doesn't need a lot of horsepower. For example, they perfectly fit the need of self-serve scanners at a grocery store. Hotel and car rental chains make excellent use of them as well for their front desks. These three examples don't make use of VMs, but they were our biggest customers.
The biggest problem with using TCs and VMs is the licensing costs. The hardware needs an OS, and often times that's an embedded Windows. It needs to be remotely manageable, and that's usually Altiris. Their are Debian-based OSs available, but it still takes time for developers to customize it, secure it, etc., and those costs are built into the device. Now let's say you want to use VMs. You need servers, OSs, the VM software itself, and support contracts. The last three require yearly fees. If you're using TCs and VMs, it's going to be expensive.
Security is the main selling point of these devices, but security is also one of the least appreciated selling points for computer hardware. Speed, features, and price are the main selling points for computer hardware, with price being the heaviest factor.