MAC paints, while they do work well as bases, are not specifically intended as such. MAC paints are a creme shadow, and can be used in whatever manner you see fit. There are no hard rules that state the you have to use a product in a certain manner. Sure, there might be some folk who will tell you that by no means can you wear bare canvas as anything but a base, but there are also folk who will tell you that you should never go out without lipliner, lipstick, and lipgloss applied. Doesn't mean they are the end-all-be-all of makeup knowledge. As long as you stay within product safety guidelines (or don't, some choose to not follow them) wear products in whatever manner YOU feel appropriate. Heck, you don't have to wear makeup at all if you don't want to. It's up to you.