I think you can actually make it so that you do get notifs when they do things? I'm pretty sure that was something in the last update, I'm not sure if that can be done now though
In IPS it used to notify you of profile posts, thread posts, etc., all as options you could turn on or off individually. XF following, on the other hand, has always been completely useless ¯\_(ツ)_/¯
On the old site it did that. But with the current version it only does one thing. When the person you follow is online you can see them listed at the home page above all the names of people online.