It really depends on what you mean by really good. I agree that you can't be really good at many things - but only relative to yourself. Meaning that if I (personally) practice Skill A for a week versus practicing Skills A,B and C in the same week I would almost certainly be better at Skill A.

But if you work on Skills A,B and C for a week and another dude works on Skill A for a week there's no guarantee that you're going to be worse than him. Depends on smarts, I guess.

I guess it's academic but inexplicably interesting to me.