Well that's precisely what the problem is. It's nothing specifically to do with CLIENT_STORAGE_BIT itself, it could have been about
Also, from reading the issue it sounds very much like they didn't really want to added it, but had to. Granted, since "they" are in charge of the extension, I have no idea why they would be forced to add something they didn't want.
It seems to me that if behaviour can't be specified precisely, then it's better off not being specified at all. I've no particular desire for CLIENT_STORAGE_BIT to mean that the buffer storage is allocated in client memory; that's irrelevant. I have a desire for specified functionality to mean something specific, and put an end to the merry-go-round of "well it doesn't matter what hints you set, the driver's just going to do it's own thing anyway". If that's going to be the way things are then why even have usage bits
In practice, applications will still get it wrong (like setting it all the time or never setting it at all, for example), implementations will still have to second guess applications and end up full of heuristics to figure out where to put data and gobs of code to move things around based on what applications do, and eventually it'll make no difference whether applications set it or not.