We just acquired our 3rd dinghy.  We've always carried a spare dinghy in the form of our 14'
Porta-Bote.  This new boat is intended to replace the PB, but be more functional.  In the time we've owned it, about 2 years, I can recall unfolding the Porta-Bote exactly three times.  It's a little bit of a major pain in the ass.  Yes, it folds to 4" flat and straps to the lifelines very conveniently, but the seats, oars, and transom still have to be stowed somewhere, and they are not small.

The new dinghy is a 10' Avon inflatable with a high pressure air floor.  It will roll up and fit into a space smaller than that taken by the PB's seats.

The driving factor here is that we're no longer a one dinghy family.  With Rachel's birth we've exceeded the rated capacity of our 12' RIB, which really only matters in places that care about that sort of thing (ie. the US).  More importantly, we have growing boys that are ready for a little more freedom.  We already let them go explore alone by kayak, and this new boat is intended to be the "kids" boat and extend their range.  As the boys grow in size and appetite, we're also expecting this boat will help them contribute to the dinner table in the form of fish and lobster.  

Another consideration is that we've felt constrained in some of our dinghy excursions in remote areas by the prospect of engine failure.  With this second dinghy, we'll have a built-in buddy boat.  Both of these uses require the backup boat to be readily available, which the PB is not.  We're expecting the air floor dinghy will be easier to deploy and spend more time in readiness.

Of course, this second boat will need a driver.  Aaron has recently received his Florida Boating Safety Education I.D. Card and is now a legal operator in Florida.