If I need to hand build a yard, I prefer to use TS12 as it handles fixed track pieces better than any later version of Trainz. (The fixed track pieces get replaced with regular track when finished.)
I also frequently get nostalgic about old routes and old content. If I want to run Canadian GP9's on Highland Valley, the '09 version of Trainz lets me do it.
Having said that, most of my larger routes won't work in old versions of Trainz. Most of my stuff is migrating to 19.