Forstall has done a lot for Apple. Remember he's been working for Apple for a long time. Aqua wouldn't have existed in the shape it was released in if it wasn't for him. iOS would have looked nothing like it does today if it wasn't for him. iOS has to a large extent been responsible for the massive success the iPhone has seen.
He hasn't done as much lately though. I do think Apple's timing here seems about right. He seems to have been very skilled in coming up with simple, yet attractive and usable interfaces. But I think he is best suited when it comes to spearheading new projects rather than maintaining them.
I think Apple may have been thinking that they do not need a "Forstall" in the foreseeable future, because Apple now works in the desktop, laptop, tablet, mobile, mp3 player, and TV spaces. They are established in the areas they want to be now. There is still much to do, but these efforts do not involve spearheading entirely new devices, new operating systems. Forstall gave Apple much of their brand identity in the software space, but they have it now. They've seen now what he could give them. Forstall hasn't had much more to give, as seen by the last two years of iOS "development"... It's been more about apps lately than building new platforms, which I think is what Forstall excelled at.
It's now more about maturing the operating systems on their devices, coming up with innovative new features, and staying at the forefront. Given that they may then start to suffer more from the downsides from his assertive team leadership style than the upsides from his innovative ideas on taking the "Apple experience" to new fields, they are better off with restructuring.
This is at least what I'm thinking here.