Clicking an Undo button is faster for you than Ctrl-Z?
I find clicking buttons much slower. Especially these days when monitors can be 40" or bigger and you have to drag your mouse across the whole screen to find a little button.
How do you work? I would normally expect our users to work with the left hand on the keyboard and the right hand on the mouse (or reverse for left-handed users), so Ctrl-Z is easily accessible.
It wouldn’t be much work to add Undo/Redo buttons. The tricky part is how to fit them into the UI design. They would probably have to live in some kind of “Ribbon”… and I’m not a big fan of the Ribbon UI concept. I find it uses up a lot of screen estate for not much benefit.