I dunno Cursuswalker - maybe there are/were deposits, maybe there aren't/weren't :-) Maybe there were so few of the bluestones in these (possible) deposits that they were indeed highly valued and all taken away. Then again, maybe you're right and they were all brought from Preseli (which incidentally is what I also believe but don't have the necessary knowledge to back up that belief).

Maybe, as I suggested above, it's a combination of transportation factors. At this stage no-one seems to be able to actually prove anything one way or the other - which always makes for good debate ;-)