may be you miss that it's possible to include css file and all will be the same as HTML. Of cource not all properties it's possible to set in css, but quit enough.
Also it's possible to define special classes for components.
I did not know there was a <mx:Style> tag, that is rather cool. Rather than looking at MXML as a replacement I guess it should be viewed as an alternative? The Flex Builder (which the name should stay the same! Flash Builder sounds... misguiding ) is the best pro for MXML.