what agencies does this?!?
The NSS-CDS for one. Maybe the NACD as well, I'm not sure about that.
My recollection of the history of this is: originally, there was no training progression in cave training, it was all done in one big lump, from what's now known as cavern through full cave. It was later recognized that the training would work out better if it were possible to progress in manageable chunks. So along with the prospect of sending students with incomplete training out for a time without instructor supervision, came the question of how to establish limits on the divers to keep them within situations that meet the level of training they do have.
With cavern certification, they're confined to the part of the overhead where daylight is still visible. This is a permanent certification.
For the first level of actual "cave" certification, the student is allowed to go past the daylight zone, under very limited circumstances-- no navigation decisions, and limited penetration. To limit penetration distance, the student was restricted to a single tank with two first stages on a Y or H valve. And James' harangue on how stupid, useless, etc. notwithstanding-- Exley and the other guys that wrote the book did a lot of cave diving with a single tank, and I've never heard that the cause of any accident was "dove a single tank with an H-valve".
So that Intro certification was permanent too. But there were students who really didn't want to do training in a single tank, and it was later allowed to do Intro in doubles, but penetration was still limited to the amount of gas you'd be able to use in a single tank. However, it was
not considered a good idea to make the doubles certification permanent, since that would increase the temptation to take a bare minimum of training and stop, and then go on to do more advanced diving without ever getting real training for it.
So then it must have seemed harsh to certify someone as a cave diver, but then strip the certification away entirely if they certified in doubles and let the cert expire, especially when the single tank certification is permanent. So, technically if the doubles cert expires, you are bumped back to the permanent singles certification.
I think all this should be considered in relation to the original premise that the intermediate levels of cave training weren't intended to be complete within themselves. They're intended as stopping places along the way to give the student a chance to absorb the new knowledge, practice what they learned, rest and heal muscles, go back to work for a while, etc. It's considered a risk for someone to get just a little training and then stop, but continue cave diving. There are numerous cases of people doing that that then go on to do dives way beyond their training that get themselves killed. Once someone starts the training, they are encouraged to finish.