The problem of not being a complete N900 device (no accelerometers, sensors and so on) is a good point. Nevertheless, most of the points are still valid.

Take, as examples, Java MIDP emulators, Windows Pocket PC emulators and Palm emulators. All those emulators didn't have touch screen, sensors, and network hardware just like the original devices, but they did have mappings for all those: regular emulator menu controls would control how the emulated hardware would "perceive" actual sensor data. Say you want the light sensor to show 100% light, just move the emulator light sensor slider to full 100%. The same mechanism can be used for all other sensors and hardware, some will be more difficult than others, of course.

In any case, there could be scripting to help change sensor data while running an application. This way you could test how you application behaves when a call is coming, etc. It would be possible to test difficult to test or expensive use cases. Suppose you are developing an app that logs international roaming call costs and durations. You could just script the sensors to show roaming to another country and start and end some calls.

As for the Nokia RDA and Device Anywhere, I didn't see N900 listed there yet. However, wouldn't an emulator be more flexible? Since the real devices already have some features blocked or not available (audio, GPS signals, outgoing calls, SMS, etc) the emulator could even be considered "better" in that it could at list simulate all events with simulated data.