The thing about the less-effective-when-against-
the-glass wouldn't really be much of an issue 
when the only space at issue is the width of a 
mat. Say, for instance, you were dealing with a 
room whose window was UV-protected. Something up 
against that window would be more at risk than 
something across the room from it, because the 
added space gives what UV *does* get through 
additional chances to spread out, while it's very 
concentrated right at the window itself.

The mat would partly be for aesthetic reasons. 
The other part would be to protect the sketch 
itself. If you framed it without a sketch, the 
graphite's going to smudge from contact with 
glass, and the glass is going to get smudged from 
contact with sketch. Heat may also be a problem 
over time... but the smudging is going to be the 
main issue that crops up.

Good luck,
-Cres