[rjknight]

OK. If you showed me an advertising poster you really liked, I could fire up Illustrator and give you a decent replica in a couple of hours. Does that mean you should hire me as your new ad agency?

No, but if you just wrote Illustrator and there's nothing else like it on the market, I might want to buy a copy.

[giveitago]

But then you've dramatically shifted from having advertising design done for you to buying a highly technical tool that allows you to create illustrations. If you are someone with no design or illustration experience, that isn't going to work out so great for you.