I was just considering this, as i'm about to get back into a G2 system and still have some 46mm filters from a Leica kit i'm selling.... But, as i thought about it, i've decided NOT to use any protective filters on the G2 lenses. I'll primarily be using the 45mm lens, and as it's priced so inexpensively, i'm just not going to worry about it. I do use filters on all of my other glass (Hassy, Canon, ContaxN), but i don't think i'm going to be concerned with protecting a $200 lens, especially since i've never damaged a lens (or filter) in the past. My philosophy about insurance/protective measures is "always protect what you cannot afford to replace." I suppose that with some of the more pricey lenses one might want to take extra precautions, but i'm likely to only use a 35+50 and possibly 90mm kit, and i'm just going to throw caution to the wind.
In a general sense, though, i think one would be hard-pressed to ever discern the difference in imagery made with versus without a high-quality filter. I would, though, only buy the best glass, and multi-coated where possible. B+W, Heliopan, or Contax Multicoated. Contax does make 46mm UV filters, "glass" and "multicoated" versions, but i haven't found images of them, so i don't know what the finish is.