I've done that for the first (and only so far) exhibit I've done. It was in a café and I decided that I was not going to spend all that money to put prints under glass. What I did was I simply glued selenium-toned RC prints over foamcore. I reasoned that the prints would get attacked anyway by particles in the air, so even a glass wouldn't make a big difference.
On the other hand, when I dropped by the Stephen Bulger gallery in Toronto once, I saw B&W prints by Volker Seding that were framed without glass. They were the finest fibre prints, slight warmtone, and were catching the light beautifully without the glass. Of course, their environment is well controlled.