Technically, you are correct. Those e-certs aren't supposed to be transferrable. But if you invited some out of town friends over and put them up at the local Marriott using these, you could easily check-in for them and they could still use the room.

And while "technically" they are supposed to be used by you, others have posted that if you make the reservation in your name, but add their name to the reservation, most hotels seem to have no problems honoring the certs. But you are correct in that a particular hotel management/staff could actually try to enforce the rule and only allow you to check-in. And while not too many will probably do that, there is a chance.