I am not convinced that this would be generally useful.
Can you give an example usecase (preferably for content on xenforo.com) where this could be useful to me?
Simple, this feature makes sense in an information discussion. Not in any discussion. In a computer forum it would be used A LOT. In a finance forum (like ours) it would be used A LOT.
When someone opens a discussion, hundreds if not thousands participate, in the chaos of responses one loses focus and interest in understanding the real problem. By following specific comments, you will be able to follow the tree of the progress of your real interest discussion-responses instead of following and receiving a notification every time for any response (sometimes even useless) from any X users.
This is mostly a support forum, so when someone opens a thread it's for themselves and it usually ends in a few posts and in a long time, because no replies arrive. End. (just an instant example below)
So, You cannot compare this Xenforo forum with others forum needs.
In our forum we have 2000 messages per single thread at least, the chaos is very much, so a specific "follow" function will be very very useful.
The same thing will be useful in a forum like Tom'sHW or HW Upgrade or similar.