I think the general perception is that there are more available parts for the Japanese motors readily available, both new, used and NOS. The price point on said parts is a consideration also, when comparing to BMW and Ducati.
Used throttle cable I bought over the weekend cost me 10 bucks. Had my choice of 3, and I've got a Suzuki GS850, a bike that is generally considered a little harder to find parts for compared to other Suzuki models.
You will find a ton of used Hondas on Craigslist and Ebay, at least here in the states. I would say after that Suzuki, Yamaha, and Kawasaki are all about in the same range tied for second, and the European bikes all lumped together in a very distant third.
I might recommend doing a parts search on a parts distribution site for a fairly common part, say a carb rebuild kit. See how much those Beemer kits are compared to the Japanese makes. I think this might help influence your choice, is budget is a consideration. And in this current economy, why wouldn't it be?
louidest143