Well, thing is just becasue everybody does something dont make it right choise of tool.
Im not uderestimating flash, i used to work on the damn thing for year.
The GUI of the damn thing is horrible. Try to simply key around the contoll points of your curve, then your suddenly have a hell of a time doing so. In maya you just move them about and key. Blim thats it. Its not that you realy need this sort of stuff as much in 2d animation but it sure as hell is very handy when doing some flowing objects like cloth outlines, hair , flags, waves and so on. Even the firgging tween eingene of flash is sometimes so unneccerily hard to operate. Now if you were to do the same things in maya youd be done by the time the flash user has time to start.
Then why the hell does flash not have a deecent graph editor for moving the keyframed stuff. This would make some of the everyday animation tasks so much quicker to do.
In the end im looking past what the program can do and achive with work, im looking at what kind of productuion pipe you could build on top of the thing. So yes it could do simpsons like stuff but remeber so can pencil and paper. The real quastion is how quickly you could work. so if i can use maya to speed up a a task by 10% i can sell the end results cheaper, or i could use the time to make the animation look beter. remeber many of the animations you see on tv today.
Try using maya frame by frame youd be supprised. Its just that its not wery often done and certainly theres no tutorials on how to do so. Its just one of thiose things people dont think about.
Problem is that flash is okay, its jut not fabulous. Mayas much closer to being that, altough mayas more painfull to get into many aspects of life. Flash is missing much of the essential toolsets maya users have. AND dont forget that being able to quicly fascuimilre bvackdrops and manequins for drawing helpers is actualy sometimes a real boon.