+1
Sometimes I go "review blind" reading all the various impressions people have, and when their descriptions contradict one another it drives me batty! Being able to hear a bunch of headphones, even ones I'm not interested in (e.g. bass monsters and highly-colored sets), would be excellent because it would give me a lot of frames of reference when reading reviews.
For instance, if somebody says that a particular headphone has slightly less midbass than, say, an HD650, I can only vaguely imagine what that sounds like because I've never heard an HD650. I'm not interested in ever buying one, so I won't be ordering one just to find out. But I'd like to know what it sounds like for reference because it's frequently used as a comparison. Similarly, if somebody says that an IEM's bass is somewhere between the IE8 and the RE0, again, no clue. All I have are my preferences (somewhat toward analytical) to go by, so I can only guess that I'd like something closer to the RE0 than the IE8.
Getting to hear a bunch of headphones at once would let me ask people more fine-grained questions (e.g. the DT880 has a little too little bass weight, and the M50 had too much, and the SRH440 is about right but doesn't have enough extension--where does the bass on headphone "X" fall?), and if they haven't heard any of the headphones I ask about first, I can substitute other ones until we find something we've both heard.