[Marc]

Originally Posted by CharlieO

That is a fair point. However I will explain why it does not work.

A thread is far more easily quotable.
Also if you PM someone they just accuse you of 'baiting' and 'stalking' on the basis that they cannot ignore a PM. If someone wishes to leave a thread and not reply they can easily do so. A PM can not be left unopened unless you want an outstanding notification and not readily ignored.
A thread would work better as one can't be accused of spamming an inbox.

To be honest you can ignore a thread or report any nuisances just as much as you can with a thread. And you can delete them if you want to leave the conversation. To be completely honest, as much as your arguments with half the forum would be entertaining for everybody else to read it would only cause arguments and sides being taken and for tensions to build and spill over into other threads by people who aren't even involved in the original argument.