I shoot with a CPL outdoors all the time. Do I *need* one? No, but neither do I *need* all that reflective light that hampers my image, of which there is a lot; more than most people realize (see
this video regarding CPL's). If you want something for protective purposes the CPL does that just fine, in my opinion, or you can switch to something like the Hoya HD Protector. I like the Hoya HD line of filters because the glass is very, very strong and the filters themselves are excellent.
If I did all my shooting in a studio I wouldn't bother with a CPL, but that's now how I shoot. The CPL protects my lens not so much from impact damage as from things like salt spray and sand (desert sand out here in Cali can be as fine as talcum powder) because I shoot at beaches and in the desert frequently. Shooting in such locations the CPL also works like a very light ND filter allowing me to use wider apertures for shallower depth of field.
There's all sorts of stuff that can happen to a lens besides impact damage... Stuff I'd rather clean off a $100 filter than I would a $1,000 lens. The filter I can clean with running water out of the kitchen faucet, the lens is another matter. What filter you use, and if you use one at all, should be based on your shooting and what you think is prudent under those conditions.
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