"I'm sufficiently accustomed to having events act on the window currently under the mouse that I'm constantly closing the wrong window or otherwise taking actions on the wrong place."
By typing a keyboard shortcut while the mouse is above a background window?
"Another massive benefit of FFM is the ability to type on a (largely obscured) window while reading data from an unobscured, but unfocused, foreground window. The desktop contortions you have to go through to do this without FFM pretty much suck."
Good point, although I think you're slightly overstating the case ;-)
One loose thought: both of these are "(moderately) advanced user" things if you think about it. In the first, you need to have memorised enough shortcuts that they are part of your daily routine. In the second you need to be familiar enough with the apps, etc., that a small portion of one poking out somewhere is enough for you to know what it is; you also need to be familiar enough with the behaviour of the application without clues from the rest of the GUI for the application.
Apple has to look after newbie consumers first and advanced users second. I like FFM up to a point (and only up to a point), but it may not in Apple's interest unless they want to evoke yet another "transition" and much less likely as long as you could argue that its more suited to "advanced" users than beginners.
What might be interesting is something that can work on top of the existing scheme so that its workable for both newcomers and experienced folk... I wonder if any of the old disability support keyboard stuff could be re-worked to acheive the same outcomes as the FFM approach but via the keyboard? (This way mouse behaviour remains the same. It'd also would have the advantage of not having to leave the keyboard for the mouse.) Idle thoughts...
While this topic has been discussed, I noticed that out of habit I move the mouse away from a textbox, etc., once its selected so that I can cleanly view what I'm typing without the pointer getting in the way.