Filter for 24-70mm

I reduced the number of my lenses to three, all Nikon: 12-24, 24-70 and 70-200. Each one is equipped with its own NC filter. Total investment 3 x $49, less than $150 and definitely less than replacement of the front element of the lens. Also have CPL, ND and Grad ND, one of each and use them occasionally. So far haven't noticed any IQ degradation with NC filter. Actually I tried to produce a flare but couldn't force my filters to do so. If the lens itself didn't produce the flare, the filter didn't add to it. There must be a reason why Nikon keep making NC.