BMAH allows players with a lot of gold to have a slight advantage over others. Items that were once thought off as great achievements have stopped being special. For instance ashes of al'ar was a rare mount on most servers even in wotlk, as were invincible/mimiron even in cata. Now they can be bought for gold, which just takes away the good feeling of the mount. Even if people were to do icc/ulduar etc to try get these, the chance was very little.
Notice how something that was special is no longer so.
Similar argument can be made for gear on BMAH.
However BMAH also sells current gear. This again is a very slight advantage to people with more money. Now, I do not play WoW and am not complaining, I just had this thought while looking at the new BMAH items list today (http://www.wowdb.com/items?filter-source=128).
Could BMAH be a sort of test to see how far they can push the envelope in order to one day bring a RMAH kind of system for items. If blizz were to get people to stop thinking of the items as something to be accomplished and start looking at them as something available for purchase, this might be a good way to do it. There would be quite a few people in wow who would not mind shelling a few bucks for mimiron head or some other exotic mount.
I am not suggesting it would be for relevant gear but for everything else, especially mounts/pets/older gear.
One also has to consider that eventually wow will slow down (at some point in future) and blizzard will need more revenue and this means looking for more revenue sources and given the popularity of wow and the generally good response of wow community to anything blizz puts on sale, blizz must have surely considered such an idea.
Would you support a RMAH in WoW which sells everything else except current expansion items?