Don't want to get into an argument here boys & girls, me being a new member and all, but it is an incorrect assumption that pressure treatment protects the whole length of timber.
Even in a vacume, or double vacume (vac-vac process) you are not guaranteed to permiate 100% of the wood - you can only introduce the treatment to parts of the timber that allow it. For example, the denser heartwood of the tree may prove impossible to permiate due to the waste deposits that are already in the fibres of the wood physically preventing it. This leaves all wood vulnerable when reworked, and the end grain inparticular. This is why the majority of treatment providers will advise the use of a cut end preservative, and will also advise against the reworking of a peice without doing something similar.
If you still need convincing let me know and I will dig out some factsheets from work.