Until GPS arrived, I had no choice on my last boat but to use a compass, as my DECCA (I am sure that some of you will remember it!) was fine, so long as I didn't run the engine!!!
In the end, having spent several hundred pounds trying to suppress the electrics/HT leads etc., I had to admit defeat and sell the DECCA set.
I have always had a magnetic compass for back up (and the occasional cross-check) but I have found electronic compasses much more convenient. My favourite was an Azimuth compass designed, I believe, for military use, so a pretty tough and reliable piece of kit. Unfortunately, I can't accommodate one on my Scorpion, so I can only have the magnetic compass.
My last boat did not have a chart plotter but I did get a Furuno GPS after a while. So why have a compass as well? In addition to the obvious need for a back-up navigation tool, I found that I could approach a turning point/waypoint at 30kts (OK, so it was a slow boat relative to a RIB, but quite quick in cruising terms) using the GPS and then turn the boat on the compass, before the GPS would catch up.
After all, a GPS can only tell you where you have been and will assume that you are going in a straight line, whereas a compass will tell you are going!
This method of navigation was especially useful around shallow areas, such as the Shingles, where if I waited for the GPS to catch up, I would either have gone aground or had to slow down. I never saw the latter as much fun!
Back to the original question - an electronic handheld. I had one of these as well as the two fixed compasses - so not too much chance of getting lost!!! I guess they would be of limited value on a RIB, unless you were stationery and needed to triangulate your position. Of course, you will need to have paper charts for this to really be much help, but then we all carry these, don't we?!