This topic comes up ever so often. Good set of reasons here:
Fat Distribution Decision
We do pass some special compile time flags to some of the builds, and spent sometime getting SSL to play nice across all the pkgs. The only patches we add are to Cyrus-SASL though. The biggest problem with using existing pkgs is the ability to reconfigure those would involve major changes to our various scripts and admin utilities.
In the end it's something we want to do but it's lower down on the list today.