For sale Pentax DA * 12-24 mm

With such a wide lens vignetting may be a problem, so any filter would have to be very slim.

I wouldn't put another piece of glass in front of such a fine lens - it will only cause a reduction in image quality, even if only because the relatively exposed filter glass will be quite prone to flare.

That is true john. Also putting a cheap filter would slow the lens down as mentioned here on Hoya's webpage.
Lens hood would be an option for lens protection but it means bigger lens size

I don't use a filter on mine (except the odd ND grad for which I have a special wide angle holder) but it never goes anywhere without the hood. With such a prominent front element I hink a hood is needed with this lens more than any other I own.