The only reason I can think of is that if you use tablets, there is no way of installing the mod on them. So you need to use plain 2.1.1 if you have to use tablets.
Otherwise, using the TSN mod does no harm and gets them ready for joining shifts.
Having said which, I keep a folder with the TSN mod installed, a folder of the latest version of Artemis and a folder with a vanilla 2.1.1 install so that I can jump into any kind of game.