I might be troubleshooting something in our network where I have no interest in making the results public. So I value the option to have non-public measurements.
There's no "right to see all measurements" here - if someone wants to see something, they are free to run their own measurements with their own credits. What I do with my credits (which do not come for free) and who can see the results should be my decision.
The problem with non-public measurements for the RIPE NCC (at least, when I was still working there, but I doubt the situation has changed), is that keeping track of whether a measurement result is public or not causes significant code complexity. So the question is really whether the relatively small amount of use that is made of non-public measurements is worth the cost to RIPE NCC members for the ongoing maintainance of this feature. Without non-public measurements, measurement results could be stored anywhere, meta-data of measurements could be stored aynwhere. With non-public measurrements, some measurement results have to stored such that they are non generally accessible. Of course API endpoints do need access to that data. At some point there is just too much code that needs to know about non-public measurements.