Well, what you said might be true, but that's exactly the point. If two of these perform almost exactly like a R9 290, then 2 of these would need the 4gb of ram...
I know you will probably say that "then, you're better off just buying the 290", but many people prefer to (or are only able to) buy 1 cheaper card now, and upgrade via crossfire in the future.
For all that group, it's a lot better to buy a 4gb cheaper card, if available, as it opens you up to better crossfire in a future...
And that stuff about crossfire broken at higher res/multiple screens, that's only temporal till they give us newer drivers.