"How many key cards would you like?" Many solo business travellers just ask for one, but here's why you should say "two".
For a start, many hotel rooms require one of those card keys to activate the room power -- and the air conditioning.
You may not want to remove that key when leaving the room (especially in hotels in hot climates where the aircon is a little anaemic), so it's useful to have a second one to take with you so that you can return to a wall of cool air when you get back.
(Of course, some of those key slots will activate with a thick business card instead of an actual key. That's very useful if you're prone to forgetting your key card.)
Forgetting your key card is the other reason to have a spare: that frustrating feeling when, caffeine-deprived, you head down for breakfast and hear the solid click of the door closing behind you -- with the key inside.
Sure, you can get another one from reception, but you'll have to queue with everyone else, during a prime time when others are trying to check out.
But if you have two keys, one for the power slot and one kept safely in your wallet, you'll avoid having to head down to the front desk to sort out a replacement key.