To my mind, Flash Builder has two main capabilities:
1) it is GUI oriented (through Flex, an extension of the Flash library)
2) it is a programming tool for developpers
By 1), I mean it provides capabilities to very quickly create menus, buttons, windows, and things like that. More interesting, Flex helps greatly when the screen is resized. With Flash, you have to process window/screen resize by hand if you don't want a mere scale, but Flex allows more possibilities by providing containers to place components.
According to that, is Flash Builder useful to develop a game? I would say no. Well, actually, it depends: sometimes, you would like a clean configuration menu, dialog boxes and button and so on, so Flash Builder could help. But for a pure graphical point of view, it is not very useful.
With 2), Flash Builder provides tools to quickly code. But you have to learn these tools. Code completion, code management, correction as you type your code, automatic compilation, "Easy" integration with source code control with Eclipse plugins, ... these are tools for developpers.
You do not really need MXML. It can provide some useful tools, like states (you define a "start" state, a "game" state, a "game over" state"), but it is not so difficult to have that with Flash Pro by loading/removing some movie clips or SWF files.
About exporting a FLA project for Flash Builder: beware, Flash Builder cannot work with the timeline and the graphics from Flash Pro. You can use them. But you will still need Flash Pro to modify them. Flash Builder can help to type your Flash Pro code, but that is all.
Personnaly, I use both. Flash Pro is used to create animations, to draw some graphical elements, to set them in the screen. Flash Builder is used to quickly type my code, to add some GUI components.
But it is not straightforward: you have to learn how both tools can interact, and the documentation is sometimes confusing. You must have a kind of well defined methodology.
For very graphical games, my conclusion would be to stuck with Flash Pro. The CS5 version has better code completion capabilities than CS4: it is not at the level of Flash Builder, but it already is of a great help. Flash Builder is rather for full time developpers.