I've always found this the opposite of useful. If someone is too busy to visit, then that's exactly when they most need to stay in touch by getting their notifications. If they want to cut the number they are getting they should be able to visit to do that, and that might reconnect them.
If notifications fall away, then a busy person is much more likely to forget all about the forum. Ive done it myself with lots of forums when I stop getting notifications. I do tend to visit when I go on getting a series of them and I feel I'm still part of it. People forgetting all about our forums is the last thing we want.
I appreciate some other admins may not see it this way for whatever reason, but that's their business. I think this block on notifications should be at admin discretion so both needs can be happy.
So how do I disable the current setting? I assume there's a bit of php somewhere that sets the number of notifications sent before the notification is disabled. So if that was commented out, or set to zero, that would get what I want, and notifications would go on sending.