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XRenderAccel requires very little support. Both major vendors drivers should work fine with it.
What about those people with graphics cards not by the two major vendors (which is just about everyone)?
Would you rather have highly flexible and powerful applications which look good or ugly ones with a weak API that run blazingly fast?
That depends which way round your putting that, because both sides of the coin there are wrong. I'd rather have efficient applications work on the widest variety of hardware possible so people could actually use an open source desktop. I think you're missing the goal here.