I wasn't saying it wasn't worth it, only wondering about its effectiveness. But thinking again, I think it would probably effective because they do send search teams when there are news of lost divers and in that case it wouldn't be a "mysterious" signal somewhere in the water, people would also know that divers were missing.
They would know where the divers were to a distance of 50m as the PLB GPS is that accurate (if you get one with GPS), they also know who has it, and if you have done pre planning and advised them on their website, they know where you are and what activity you are doing. Thus if you inform them you are likely to be diving in Bali from 10-20 Jan 2014, and the signal is initiated in Crystal Bay area, they will take it to be serious.
Tested this out accidentally when my PLB initiated by accident. On ringing the Rescue centre after a phone call from them, the chopper was being fired up as I spoke, even though they could see I was potentially on land. I know being somewhere like Bali, or worse Fiji where the navy doesn't work on weekends, ones chances of being rescued are less, it still increases the chance simply by the flowing facts;
1. Someone knows you are missing, and that someone is in your country and not some corrupt official
2. They also know where you are to a position of 50m on the globe, thus there is a fair chance someone at the site you are lost may know this as well.
3. They continue to know where you are for maybe 24-48 hrs depending on battery life, and even if the battery dies, they have a good idea where you might be.
It simply increases the odds for you. Can you afford the $600-$1000 for one? If you do serious diving in risky areas, I can't see why one wouldn't spend the money. Diving is a high risk sport, and diving in high risk areas increases that risk. Having a PLB somewhat reduces part of the risk. It wont stop you from being pulled down to 200m with a vicious down current, wont stop you from the bends but will give you a fighting chance if the boat disappears or if you get carried away. having been in both these positions before, I can tell you without a doubt that in your left BCD pocket is a little item that just might get your butt back on dry land.
But I do take your point, its not a guarantee of rescue, it just gives you a better chance than if you have nothing.