Don't think it's 90% for laying out the elements. I tend to quickly layout objects in mxml, then do all the logic in AS, addEventListeners etc..etc..
you can create all your elements in AS if you want, and do addElement or what have you when you want it on the stage but it's archaic.
Have you looked at Catalyst?
Thanks Spaghetti, I guess my concern is that yes placing assets with AS is a pain, but most of what I do is heavy animation, transitions, effects, 3D, arrays and istantiation, etc. (in other words a lot more than just button logic). So I'm not inherently against MXML, but everything I've read so far about it leans more towards making more-or-less static pages that get navigated fowards and backwards through linearly. But I could be wrong - I would love to learn how to leverage MXML and Builder's visual design functions if it lets me use that as a no-strings-attached starting place for otherwise complex, entirely-AS Flash, but not if I have to use AS in-line, or if keeping the code separated causes any problems whatsoever (which I have heard of problems with, but I don't know if those are just peoples' errors, or if there are actual limitations with code-behind).
I haven't looked closely at Catalyst, I figured it was just kind of what the Flash IDE should be for designers in the first place - will that allow me to start with pre-laid-out assets for scripting in Builder? That's really the only step, or bridge, I am looking for here, or need. I really just am hoping to use builder as a better environment for AS coding.