Explain to me how having two extra spaces on my hotbar does me any good when there's nothing to put into those spaces. This all sounds like massive paranoia over a problem that doesn't even exist right now because we don't even have new skills. This may all sound good in theory, but just as much as you say it's useless to keep them as separate buttons, so too, is it pretty damn useless to condense them into one right now.
It's bad design to have excessive white space in a graphic and nothing to put it in.
Furthermore, excessive buttons are only, only bad design on a web page when they don't DO anything or GO anywhere. When they serve no purpose. The same can be said for all your other myriad examples. Wait...did you just use a switchboard? They're suppose to be like that.