I agree that the price is right. Personally, I still have reservations about purchasing expensive REs in which I don't receive the VST/RTAS/AU equivalent (or a cross-grade option to do such—the same as many folks would like a cross-grade option moving into RE). I'm not saying the price is necessarily wrong, but for me as a consumer, I'm not comfortable making an investment that locks me in (meanwhile, in the plugin world, when you purchase a plugin you usually receive several formats that you can use across countless DAWs). However, that's my opinion and it's not related to this idea.
This idea of renting would be optional to developers. Of course, a RE that costs $30 might not make a whole lot of sense to offer as a rental. But then again, if someone doesn't plan on buying the RE, has used up the trial period and wants to use it to hear someone else's project (and will not purchase the RE), it would potentially be extra dough for developers when that person rents it. Again though, the option to rent would be up to the individual developers.
In a little over a month (Aug 14), the FET Compressor will be $150 and the TSAR-1 will be $200. I've tried these and decided I do not have a need for these. However, if a buddy of mine has written a song with one and really wants me to hear the mix and collaborate on it (and doesn't want me to make a compromise by substituting the missing REs with an RV7000 and MClass Compressor), I might be inclined to rent the REs for even $20 each for a month to finish the project. Money the developers otherwise wouldn't see.