This was one of the first suggestions to be made after the release of REs, and is still a good option IMO. Another suggestion was the ability to pay for additional 30 day trials so you could open an important project but wouldn't have to pay full price for that privilege (especially if you didn't want the REs in question, but needed to collaborate).
Not having any control over an RE is the less desirable solution IMO, since an RE could be in a chain of FX and making a change to an early FX could require changing a later "open only" FX. And of course, you'll also still have to download each RE so it can load on your system in "open only" mode.
Plus, there are MANY details that would have to be worked out in the SDK for this to be effectively implemented, yet the "rental" approach requires only a small tweak to the authorization system. :-)
Giles Reaves, aka 'selig'
Audio Illusionist, Musical Technologist
Selig Audio, LLC