It saves space on the phone, but the downside is that you have to press the power button first to turn the screen on. With a dedicated fingerprint scanner like in Mix you can unlock the phone with touching just the reader. No need to mess with the mechanical buttons. But either way, opening the lock with fingerprint instead of a pattern or a pin is a big step forward.
I think there is only an optical sensor for the fingerprint in V and that uses the light from the screen. So unlocking while screen is off would require keeping the touchscreen on all the time to detect when the finger is there. I don't know how much it would shorten the battery life to keep the touchscreen constantly on.
There is something weird going on with the forum software today. There are some threads where I can see the first posting full, but all the replies are shown in a fixed size box. For example I can only see the first line of my reply above. Some other threads are working fine. Is it just me, or is it the same with everyone?