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.
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.
Good call on point 1, I confirmed your findings. I guess it was strongly on my mind since I am looking into the 16-35 II, and the reviews point out the moving/breathing/vented front element. The 70-200 2.8's don't need a filter to be weatherproof.
on #3, a sharp impact really isn't what I'm thinking of, I'm thinking more of small scrapes, dust, speck of mud, fingerprint you left/didn't see, etched onto the glass over time, you name it... I'd rather the filter take that versus the front element... easier to clean, and sacrificial if need-be.l
I do agree that it's small potatoes either way, we're really making a mountain out of a mole-hill, especially if you're already using the lens hood all of the time.