Malkoff has the following disclaimer on their site for the E1T and E2T:
"Please Note that the Optic Itself Serves as the Lens.  In Weapon Applications the Head Should be Placed Behind the Muzzle to Minimize Powder Burns on the Optic.  The Optic is Replaceable if Necessary"  
I'm just wondering how they are holding up in applications where they are placed close to the muzzle.  I mounted mine the other day and it is just slightly behind my PA Mini-AFAB comp.  If I move it further back it's difficult to use the tailcap.  I'm wondering if I will have any issues.  Obviously I don't want to damage the optic.

If they aren't putting a glass lens on the front, those polycarbonate TIR lenses (or what ever plastic they are made out of) will lose the battle in the long run.  If you can avoid it getting direct blast, that would be ideal.  The other option would be to get either a blast diverter thing, a can, or some sort of flip cap for it.  A few shots with it exposed wouldn't be terrible, I don't think.  But shooting it tons would kill your output over time.  How quickly I don't know.  But I am sure you could talk to the guys at Malkoff as they are great people to deal with, and see how much a new lens would be, then plan accordingly.