Being an avid PLB owner, I kind of saw Doug's PSP as something to keep me comfortable and entertained while waiting for help to arrive - and to signal them when they are nearby.
Let's hope that will be the case if you ever activate your PLB. However circumstances may vary. In bad weather, your signal may get through, but rescuers will be delayed. Helos usually don't fly at night (some are equipped to do so); fog and high winds slow things down.
Ground troops can be tardy due to a whole bunch of factors. Believe it or not, not all rescue groups are equally competent, due to a whole variety of circumstances.
Signaling when in trouble is definitely a great idea!