I'm assuming the Cat. 1-5 certs you are referring to are probably certs from a CC or similar. Technically, those certs can only be used by the person who they were awarded to. That means even in the scenario you mention where you add their name to the reservation, the hotel could "technically" not let them check in until you show up. Or they could charge the other person's credit card (probably required before they will give them a room), for the night and then charge you with a no show.

I'm not saying that actually happens very often, but the hotel would technically be within the rules to do that. Note that normal "award" certs ordered by you from your points can be assigned to anyone you choose. The problem with CC and Megabonus type certs are they were "ordered" in your name and thus only you are supposed to be using them. Since you didn't "order" them, you can't "re-order" them in someone else's name.