I'm using an E52 at the moment. I have it configured so that any key answers the phone. Why? Two use cases to illustrate:

I'm at home and the phone is not in my pocket, but on the table. I'm not near the table. I'm even on another room. The phone rings and I run to the room where it is, reach the table and just before picking up the phone I just push the keyboard, any key. When the phone gets to my ear, the call is connected and the caller ready to listen. Previously I had to pick up the phone and look for the green key. Time enough to miss calls (and you bet they were missed).

I'm in the car, the phone rings: I can't look closely at the phone looking for the green key. I just push the keyboard, any key, and the call is answered. Previously, I might push the wrong key easily (sure, the E52 has crammed a lot of keys on its keyboard).

Facepalm all what you want, but put an N9 in those scenarios and you'll see answering calls would be even slower than the regular mode on an E52.