Hi.
I've recently bought a Nikkor 24 - 70mm f/2.8 ED Lens, and I definitely want to put a filter on at all times for protection.
Protection from What?
Is there blowing sand or saltwater spray? Will you be cleaning the filter many times because of the envviromant.
No filter will protect a lens from impact but a lens hood might.
When you buy a filter the best way to test it is to stand wth you back to soething bright, like an exterior window or a llamp and look at the reflection over your shoulder is if the filter were a mirror. The best filters make very poor mirrors and you will not be able to see much. Really cheep filters will reflect well and give you a good view.
So, seeing as this filter will degrade the quality of every image shoot through it yo want the one that does the least harm. The filters that make decent mirrors will lower the overall contrast and maybe even cause some lens flair. the better filters do the same but only to a nearly undetectable level.
Much better to simply use a lens cap.
It's like buying a 50" plasma TV and putting a piece of plexiglass over it
The plexiglas idea is reasonable in special cases where the LCD screen is likely to be damaged. Perhaps in a machine shop. Or maybe where small children can touch it with dirty hands. I'm sure there are many cases where you'd want it.
Same with filters. Blowing salt spray is the worst. You can't clean it off the glass except to soak the glass in water. Easy to do ig you have a filter. I've soaked slat-sprayed equipment more tan once.
But it would be silly to use a clear filter in a studio environment.