The way I look at it, it's not "yet another" screen alerting you, but more of a replacement screen, as sounds and vibrations are disabled on your phone when you're wearing the watch. The benefit of this is subtle yet drastic if you're anything like me (i.e. somewhat easily distracted) - any notification on your phone is a potential time drain as checking it can so easily lead to a few minutes' scroll through facebook or a news app etc, often before you've even realised you're doing it.
Getting notifications on your wrist, on the other hand, adds a bit of distance from your time sink apps that means when I get a text or some other alert, if I'm not responding to it immediately my wrist goes down and I'm back to what I was doing. Furthermore, a tap on the wrist and a small flick to check it is much less disruptive to concentration than a buzz in your pocket that leads to pulling your phone out for a look-see (as you never know which buzz is going to be important).
Of course this might all be irrelevant if you're an Adept of Attention but I've definitely noticed a severe drop in phone-related procrastination in the (almost) two years I've been wearing mine.
