The question is who Diablo 3 should cater to primarily, who the game should be focused and balanced around, and so forth.
Again, tuning Diablo 3 around people who mostly solo or play in small groups and don't trade is going to make the game trivially easy for those who do trade with any seriounessness. Obviously if you can beat the game fairly quickly just using drops you got from your own adventuring, then it will be extremely easy if you can trade for much better gear.
By contrast, if you balance around traders, then solo/small friend group players are going to find the game extremely difficult. The game might take months or even years to complete without trading.
And so the question is: who do you balance around? Because one of the two groups is going to lose out. Either its too easy for people who can trade, or too hard for people who can't.
I am suggesting that it is far more sensible to balance around the traders, because they are simply a more important and more substantial part of the Diablo community, and are the most important part of keeping Diablo alive for so long. Of course, this is precisely what Blizzard has done, and so I don't think there is a problem. Blizzard is correct focus more on people interested in the AH/trading and less on people who simply want to loot whore/solo play, because the former group is the primary driver behind the persistent success of Diablo 2.
I do think Blizz balanced around making profit on the RMAH. The drop rate in this game is abysmal. If you go for 10 to 20 boss run rounds with 5 stacks of NV and all you get is shit that you can't even sell on the inflated AH which is overflowing with mediocre items then it's not fun but just frustrating.