Originally Posted by MSPeconomist

The problem with doing this is that you become liable for the friends total bill, including any damage done to the room. You're probably also responsible if there are noise complaints, etc.

From the friend's point of view, you have access to the room at any time and with no warning by just showing your photo ID and asking the desk for a key card to "your" room.

If the friend's name isn't on the room, he cannot get access or another key card if he's locked out, even if the front desk agents did see you together at check in or observed you giving him the key card.