My problem is that I often forget to do so. Then I can't remember where the thread was that I responded to if I want to go back and check on it later, or even if I just want to go back and subscribe to the thread. Also, there appears to be some fixed limit as to how many threads you can subscribe to. Granted, that limit appears to be quite high.
I agree that this is not something that everybody needs. I think it would work much better as a settable user option.