Always do...and those who steal the key card...WTF!
I always check out in case there is anything I disagree with on the bill; e.g. an incorrect room tariff or room service items that someone has put against my room incorrectly. It's much harder to get it corrected after they have your money.
Last few stays for me the hotel has had a fast checkout box to put the key in. I do every time as the is always a queue if you want to speak to someone.
A fastdrop service obviously benefits the hotel / cleaning staff, but poses no real benefit to the customer I suppose, as there's still no way of checking the bill pre-check-out?
I always check out. Doesn't take a long time. A matter of handing in the room key/card.
I always check out, I once had £200 worth of drinks put on my room I never drank.