joshmurrah said:
I answered yes. I put a UV filter on every lens that will take it, and my 70-200 f/2.8L IS II is no exception.
1) Most L lenses aren't fully weatherproof without it.
2) It's easier to clean (flat glass, no ribs/ridges, you're not brushing the front element)
3) The usual protection reasons... banging it against something, sand/dust, etc.
edited to add: LOL at the cling wrap response!!
1) Isn't true, almost all L lenses that are weather sealed do not need a filter to seal them, the 16-35 MkI and II and the 17-40 are the most notable exceptions.
3) Works great in theory, until you break the comparatively flimsy filter and rub nice shards of glass on your front element.
There are very good reasons for using filters, and equally valid reasons to not use them, it really is personal preference as lenses have been protected, and ruined, going either way.
I tend to use them in very harsh conditions (I am often in salty spray and sandy conditions) but the rest of the time leave them off as I always use hoods and doing so mitigates many of the reasons people give for using filters.