It's not really "unlimited reservations" other than in theory.
What I mean by that is that you can only book your first three in advance, and then after that you can book one at a time. After you use your fourth, you may get a fifth, and so on. So, you're really limited to however many you can get through in a day, based on timing and availability.
And, since the most you'll be able to have booked at a time is three, and then it goes down to one at a time, I don't see this as being something that would be too terribly hard to keep up with.
I do agree it would be cool if they had one with a watch or a built in alarm of some time to remind people that it's time to head to their next fastpass attraction... but then again, as much as many people look at their phones today, somehow I doubt many people forget lol.