0.2 alpha makes it almost disappear. I think it's a bit too distracting. And the alpha is not actually needed - setEnabled(false) will already gray out the caption (through the skin). I think this is enough to communicate the button being disabled, while not being so distracting of a change that you notice it even when you weren't looking at the button.

We also do not use alpha at all for other disabled buttons (e.g. in settings window). I don't mind using it, but if we do then we should stay consistent, and I'm not sure if this can be made generic (i.e. add the alpha to the Disabled state in the skin)