Ask Temperance on coolminiornot.com (If he has been there since hired by WotC), or Peter Lee on forums.gleemax.com. Or ask him here on a thread.
I am sure he would tell you that painting minis is not for the purpose of speed, but to make them all look like what they represent.
Good enough for rare, and good enough for common, are both of equal quality, so the point still remains. PCs shouldn't be "perfect" and monsters just be "good enough". Likewise rare should not be "perfect" and everything else just "good enough".
That is what quality control is all about.
More often times or not, you find that most mini painters treat them all equally. Be that of poor quality, or better quality. The production line on the other hand may put more emphasis on one or the other for artificial reasons as only secondary markets would benefit from better painted rares, because the manufacturer has made its money once the item is sold to the distributor, and the distributor has made their money once the item has sold to the retailer. The retailer has made his money when the item has sold to the customer.
Only singles markets can gain from varying paint qualities, so why would WotC put any time, money, or effort, in making rares look better than commons or uncommons?
Also to note of late it seems the paint quality and resulting sculpt from the molding process has not even yielded "good enough" minis.
So as long as the quality is the same across the board, then the product line will live or die by the average quality of the product.
(addendum: DMs may not even paint PC minis, so the quality of the PCs would not be of concern to them. The DM would put his effort in his avatars for the game which is everything else.)
