This isn't as important for your personal phone, but the dev-phone comes 'pre-rooted'. Depending on which one of the Droids you are looking at, installing newer firmware may not be trivial. With a Nexus One, new firmware is supported operation, and Android 2.2 is actually supported on the Nexus One.

(The last thing you want to be doing while doing your own dev work is wondering if the camera/accelerometer/whatever API calls are failing because of a sketchy driver or whatever.)

(Even if you're not targeting right now 2.2, you still want to have the ability to test on it.)