Lightening strike, fire, loss of your antennae, loss of your electrical system all would incapacitate your DSC VHF radio (the AIS part really doesn't matter for the purpose of this discussion - any DSC radio will offer the same benefit).
The beacons are self contained - they are not subject to whatever problems the boat might be suffering from. And, they can go into the dink, raft or drink with you.
PLB's serve important additional MOB functions.
A PLB, if it has a GPS integrated, is just a smaller Epirb, not as expensive but expensive anyway (more than half the price) and the only inconvenient is a smaller time of signal emission. That can only be a problem if you sail far away from the coast and if takes a lot of time to get there.
I agree that it is better then a DSC but it is all a question of price. If price is not an objection just buy a Class one Epirb and have also a smaller PLB
A radio is needed on the boat anyway and if one wants to renew it's old radio than a DSC is better than nothing and a lot cheaper. I only talked about some having an AIS because then it is even cheaper, I mean regarding having to buy a separated AIS receiver.