Running on the emulator is good if you haven't paid the $100 to put it on device yet. After that, you better be running on your device every time. There's so much to be said for actually holding the device and using your app like your normally would, instead of pointing and clicking with a mouse. Even the pixels and text-size/color are WAY different when on the device. Seriously, do this if you aren't already.
I don't think you ALWAYS have to run on your device. Emulators are much faster at launching and getting to it. I use the emulator for a majority of quick fixes, layout changes, API debugging, etc. I move over to the device when things are nearing a point where I need to test real-world things.
Using the emulator is fine, just be sure to test on the device before you deploy.
Running on the emulator is good if you haven't paid the $100 to put it on device yet. After that, you better be running on your device every time. There's so much to be said for actually holding the device and using your app like your normally would, instead of pointing and clicking with a mouse. Even the pixels and text-size/color are WAY different when on the device. Seriously, do this if you aren't already.