Since we are speaking of the SHTP, here is what the rule currently states [per 2016];
4.46
A 406 MHz EPIRB, including a battery with expiration date no earlier than July 23, 2016, and evidence of current registration with the U.S. National
Oceanographic and Atmospheric Administration (NOAA). The MMSI number to supplied to the Race Committee. A Personal Locator Beacon (PLB) may also be carried, but it does not satisfy this requirement.
There is are very good reasons for this, particularly in a situation where you are far from shore and rescue could be more that 24hrs away.
Mostly it boils down to signal strength and duration.
Odds are not that great of a small PLB's signal being picked up on the first pass in an area of limited sat coverage, particularly if you have it partially submerged on the corner of your life jacket.
You can maybe expect your PLB battery to transmit a maximum of 24hrs; larger EPIRB's are rated to at least double that.
While there are, I suppose, scenarios where the self activation matters, it's much more likely you would be activating it yourself, whether you are abandoning to a life raft or due to injury or illness.
All of that said, I carry both... but I assume the PLB is mostly useful within Helicopter range [~100miles offshore]
FWIW.
DH