Part of the problem is also that the compositor often can't know if the frontmost window (or any window, for that matter) is 100% opaque. If your window has partial transparency, or (for example) implements non-rectangularity by using an ARGB window and setting the alpha for some pixels to 0, then unredirecting it will break that.
I suppose you could just restrict the optimization to windows with an RGB visual, though. But then you need to make sure there isn't something else modifying the opacity of the window. I've written a standalone compositor before (that is, a compositor that runs independently of the window manager, as a separate process), so you can't always assume that the compositor "knows best"; for example, the WM may want to make the window decorations semi-transparent. Or maybe the user has specified slight transparency for all their windows. Or something.
Hmm... in the end, though, you could probably come up with a heuristic that works in most cases.
I suppose you could just restrict the optimization to windows with an RGB visual, though. But then you need to make sure there isn't something else modifying the opacity of the window. I've written a standalone compositor before (that is, a compositor that runs independently of the window manager, as a separate process), so you can't always assume that the compositor "knows best"; for example, the WM may want to make the window decorations semi-transparent. Or maybe the user has specified slight transparency for all their windows. Or something.
Hmm... in the end, though, you could probably come up with a heuristic that works in most cases.