Evo
Thank PPLIR link says
The restrictions on the certificate are chiefly that flying for remuneration (as with any other PPL) is not allowed and that the FAA certificate is valid only as long as the foreign license is valid. However, there is no geographical restriction. The restricted certificate allows its holder to fly an N-registered aircraft anywhere in the world.
which suggests that one could not fly oneself on one's business on it, for example.
The other advantage of doing the full FAA PPL is that the Class 3 medical is a lot easier than the Class 2 - relevant to many "older" people.
The question that arises is why would anyone want to do an JAA IR if the FAA IR isn't worth doing unless one has an N-reg plane, and if one has an N-reg plane there is no point in a JAA IR.... I suppose it makes sense if going to become a bizjet pilot?