I noticed slight pitting on headlight just a few months after buying the bike 06. I bought some headlight cover material from Griotsgarage.com item #10228 about $40. UVB/UVA stabilized polycarbonate vinyl 40mils thick.
Have had it on for 19 months and haven't noticed any decrease in light output. Went on fairly easy and trimmed with a razor blade, most people who look at the bike don't notice the clear film.
Rogues are preferable to imbeciles because they sometimes take a rest.