What put me off was that mxlmc was really slow (10-seconds-to-compile-a-hello-world kind of slow). Also, I find that debugging HTML/CSS/JS stack is really easy with Firebug et al, whereas debugging swf's isn't quite a walk in the park.
With that being said, every development environment will have its warts. If you're sick of HTML/CSS/JS, nothing is stopping you from using whatever you want. Look at the QuakeLive guys, for example.