Their ability to access the reverse side of a pitched roof is a big advantage in towns where the street layout makes this necessary, and they can be as useful as an ALP in some narrow streets where it is difficult for the operator to angle the booms.
They are also generally a bit smaller and more maneuverable, as well as usually being quite a lot less costly to buy and maintain...
I have not heard of them being marketed here in the US yet, perhaps because we do not have the problem of having so many narrow streets, but they are showing up now in many European countries and farther afield, in the Gulf and in Turkey for example...
Added by Ian Moore on 11 October 2011.