Personally I think the PM system should stay as it is.
Amongst other things, I really don't think that many would be even slightly interested in some of the detailed information I've provided to individuals via the PM system. Also I think many would have been too embarassed to have publicly asked the questions I was answering.
This is a stocks forum primarily, but there are a LOT of things which are in some way relevant to an individual stock. But, apart from the individuals who have asked me, does anyone else really want to know the precise details of, for example, how electrolytic zinc smelters work? Those things are relevant to stocks in some way but of very limited interest to most.
If someone wants to know something then I'm happy to provide an answer but they're likely to be the only one who wants to know. It seems likely to irritate others if, for example, the ZFX thread turns into a chemistry lesson just because one person wants to know how the process actually works. Of course there is some financial benefit in knowing since such thorough knowledge enables you to understand exactly what influences their profits, but it's way off the general subject matter of a stocks forum.
I've also disclosed some personal details via the PM system and there is simply no way that I would have posted such information.
So I think it should stay as it is unless there is clear evidence of abuse in which case just ban the relevant individuals.