Usually there is no reason to do get a guest certificate.
Although that might well work sometimes, I believe it's not very good advice.
II's T&C make it clear that only the person whose name is on the II exchange confirmation can check in unless II issues a Guest Certificate. Thus, although it's easy to add a name to a Marriott reservation by simply calling Marriott, an alert check-in clerk should still ask for the Guest Certificate and, without it, deny registration to anyone other than the confirmed exchanger.
Imagine that because an exchanger wanted to save $39, the guest shows up to check in and can't. Would the savings have been worth it? Besides, it's cheating.