Some developers have asked about the support for copy-protected apps on developer devices, and indeed there is a limitation you should be aware of. Many developers are concerned about the unauthorized redistribution of their applications, so they make use of the copy-protection feature (known as "forward locking") which prevents applications from being copied off devices. However, developer phones like the ADP1 allow for unrestricted access to the device's contents, making it impossible to enforce copy protection. As a result, the Market application on such devices is not able to access copy protected apps, whether they are free or paid. If you choose to add copy protection when you upload your application to the Android Market, then you won't be able to test it on the ADP1's Android Market client. Your application will always be accessible to users who have standard configurations though, and if your application (whether it is free or paid) is not copy-protected it will appear on all devices, including developer configurations.
So I've found a reason (other than it being more expensive) why someone might want to buy the locked version of the Android phone. I still think the dev phone is more attractive, since the unlocked phone is pretty much a Linux box that fits in your pocket. (And I'm pretty sure it's more powerful than my first Linux box, too. =)