Have you ever thought about that?
I mean REALY gave it a thought?! I have spend the last couple of days pondering and reading about how the legendaries and set items are complete crap while blues/rares are BIS(best in slot). More over there have been several occasions where vendors provide absurdly overpowered rings necks and other items.
More to the point if you look at the budgeting scaling from normal to nightmare and from nightmare to hell you will realize that the progression is not arithmetical but geometrical. Aka in essence the level of the item(or tier or w/e you want to call it) MULTIPLIES with the stat it can provide for example % damage which in turn is multiplied by the DPS of the weapon. And so the scaling goes way out of proportion the higher the item thus enabling us to see blue/rare weapons with 1k+ dps and hundreds of the +%damage stat. The result is a weapon or item(to a lesser degree but still applicable) so powerful that if rolled with other affixes would be rendered completely useless.
Now lets look at the legendary items. They have a decent amount of DPS and static affixes with possibly one or two random properties. And since there is No legendary items with the top affixes in the game(because I`d wager someone @ Blizz saw what would happen if they put one with them) there is ultra slim chance for that legendary item to be better then a blue rare one. And to top that we have a pure statistical inability to roll such a legendary item compared to a blue/rare one.
But one can ask why? Its simple. Lets take a +6 random affixes craft rare. You have 6 independant rolls for your affix to appear .And due to the geometrical scaling of the DPS and affixes if you roll the best affix twice(say flat damage combined with %damage) it does not matter that much what the other 4 affixes are because you essentially have a min-maxed item. And if you fail ...well you can craft/hunt those kind of blue/rare items en mass. Now with a legendary item you are double penalized. 1) you need specific zone/difficulty to be able to have a chance to loot one say 4th act on hell 2) You are hampered by the lower drop chance compared to another item of "lower" quality. And then you have not 6 but 1 or maximum of 2 "affix rolls" that can make or break that legendary item.
Last but not least I want to touch about the budgeting of certain affixes and their power compared to the rest of stats you can have. +%damage and +flat damage have in my opinion to high of a budget from the gate. Even WITH arithmetical progression scaling they would still trump all other stats because they provide so much of the "best". Who needs life regen or survivability when you can simply blow stuff up with the huge amount of damage the characters can do with those stats stacked. Also I won't to emphasize the words of Blizzard CMs and Devs themselves claming Hell and Inferno will be abysmal until the people attempting them gear up with enough defensive stats to endure them. But that did not happened because when people grabbed those 1k dps items and stacked them with x%damage stats they did not care about survivability because stuff died before it even reached them. Don't believe me? Look at the 4 monks and 4 demon hunter builds Athene and his crew played and managed to breeze through the inferno with 10k hp and almost 0 defensive stats(Yes I understand that they relied on near invulnerability skill combination but even without it they still managed) or look at 1 tank 3 ranged player combination that is so popular in EU and Asia. People don't care about defensive stats or +magic find(to a lesser degree) because a combination of ultra overbudgeded stats trump everything else in the game.
Before I end this wall of text I want you the players and the Devs and CMs at Blizzard to think about a scenario for a moment:
In my opinion there will be a moment not so long from now when someone will look at the blue/rare vendors on Inferno and will see a 1k+ dps item or other stat- stack item and will buy his heart out. What more he will invite his friends and they will do the same. A week will pass the AH and markets will flood with this item and you will realize that 99.99% of the items and item combinations in the game will be Irrelevant. And then you will realize that what your saw in this post is the hard cold truth.