Dave,
My recommendation for a ocean crossing boat would be two EPIRB--one near the helm or companionway, and one in the survival /ditch gear in the raft. A PLB for each crew member. Yes, this is a lot of bucks--but when crossing an ocean, the risks are substantially higher.
For the C Dory type, if in remote areas, just a PLB is better than none. We have 3 because the batteries were getting toward the end of life on the two we have, and I got the new one for less than $200. We will replace the batteries on the older units this year.
I agree that the two way message of the Spot--and especially in reach, which we tested last year with one of C Brats is fantastic--and can be a really great help in an emergency. It can also re-create what happened as it did in the Enseneda race sailboat which hit North Coronado Island several years ago, and all were lost. In that case there was a failure on Spot's emergency service, and no message was passed on to SAR that a distress and lack of tracking had occurred. The other issue, was that once the spot went in the water, it failed, and there was not further tracking. The PLB or EPIRB should continue until the battery fails, or if the owner cannot hold the PLB above water. Thus the drift of a raft can be tracker.
The reason I am more adamant about small boats is from what I have been reading on the various forums for the last 10 years, of the occasional incident an example of which was a small Boston Whaler, ran out of fuel, flipped, and the owner was in the water with a floating boat--but eventually drowned/hypotherma, in the San Francisco area. If that owner had a PLB he could have put out a distress call, and his life would have been saved. I hear about similar incidents a number of times each year. There are many other tracking mechanisms---but the relevant government SAR community is notified promptly with the PLB and EPRIB. Each of us chooses out comfort level. As I age, and become less agile, and strong, it changes.
No arguments, just feelings, and beliefs.
Regards,
Bob