Just travelling north, from London (Kensal Green, currently at Cowroast) over the last 2 weeks... didn't see one "data checker" at all... plenty of volunteers about though.
I think that these "data checkers" are necessary and generally benign, as all they do is collect boat numbers and positions. Someone else then decides what to do with the data gathered and then contacts the relevant party or parties as part of the enforcement process. I have found that it's not very difficult to comply with the current CART CC'er ruling/"range" on an annual basis. I sometimes find it difficult to understand what all the fuss is about.
However, i do think that it's probably a good idea to keep a "log" of your own movements for reference incase of discrepancy. If your mobile phone has GPS (most do these days) take a photo at every mooring, then email it to yourself, shove the email into a folder and forget it. If your phone does not log your position when taking photos (as with everything these days)... there's an app for that. That way you have a complete log of your movements should you (or CART) need them or the "data checkers" manage to miss you on your travels or your boat name & number were obscured by the bushes they were hiding behind .