Oil paintings (and acrylics on canvas) are normally varnished for protection, and require periodic cleaning. (Traditionally, oils were varnished with Damar varnish, but over the past few decades a non-yellowing synthetic such as Liquitex Soluvar has become the standard; such varnishes can be removed with solvents that have little or no effect on the underlying painting. Synthetics are also available in matte formulations that don't have the problems of matte natural varnishes—they get their matte effect from waxes.) The cleaning process itself usually involves water in one way or another, which can be a problem for any water-soluble inks, dyes or grounds (such as, say, gelatine in a traditional photo emulsion).

Room air isn't just a collection of gasses; there are normally any number of aerosols (liquids and particles) suspended in it. If the picture is hung in a house, it's hung in a place where minute traces of every cooked meal are wandering around the place looking for a nice spot for a nap. There's vehicle exhaust, etc., wandering in off of the street. Dust abounds. Anything you've sprayed to clean or polish something else (including yourself) has overspray. There might not be anything that's visible or even particularly appreciable at any given moment, but over the years it all adds up. Eventually, your pictures will be covered in a film of schmutz that detracts from their appearance. So, by the way, will the matte.

Then there's the whole moisture moderation thing to consider. When pictures are framed properly, they form a package that significantly damps out fluctuations in the humidity of the surrounding air. Expansion and contraction (in absolute terms) depend, of course, on the size of the picture—a 20x30 inch picture on a natural paper ground will be a couple of millimeters larger overall at 90% humidity than it will be at, say 40%, while a 4x6 won't seem to change much at all. But the front of the picture is open to the environment without glass, while the back is protected by the mounting board. Even a small unglazed watercolor will begin to wrinkle visibly if it's not in a humidity-controlled environment. Depending on the ground (some papers are coated on both sides with a synthetic material that acts as a moisture barrier) you might see the same thing happening with your prints.

You can try alternatives, like varnishing or lamination, both of which can be had in less-glossy formulations (although I have to warn you that lamination tends to decrease the perceived "art value" among patrons). Neither, though, is a conservationally sound practice since they are irreversible. Whether or not that matters depends on the expectations of your patrons; are they buying pretty pictures with a limited decoration life, or are they buying heirloom works of art?

To sum up: there are good reason for framing works of art on paper behind glass. There are alternatives, but those alternatives also have their drawbacks. Your market; you decide.