Hello,
With FischersFritz we have a discutions about where using Multiyolygone.
Personally I relay like using multipolygon for many reason, essentially because I many times tine find some area where the polygone have some commons points, an some unwanted space or some overlapping parts, when we use multypolygone is clearer, and we make less mistakes.
An other reason ti when we have some overlapping ways (sometimes more than 6) ti becomes relay difficult to select the right way.
But the main disadvantage of using multipolygon is that's a bit more difficult to understand. than we can have some thing that that (start of the discussion): http://forum.openstreetmap.org/viewtopic.php?pid=184210#p184210
An other point is that if a multypolygon isn't complete his display can be not so good in Josm.
Than for me multypolygon should be use if we have two area with a common long border to say something with more than 10 points in a row (rules that from now I didn't respect ;-) ).
FischersFritz have you something to add ?
What's your opinion ?
CU Stéphane