A large amount of people who don't smoke marijuana now will very likely make some type of purchase of it.

"Hey, look at this thing that everyone is talking about, I'm going to try it out. Also, it's legal!"

Also, enforced laws are great and all, but underaged drinking is illegal as well, and how many minors drink each year? It's a lot of them, and marijuana will not be an exception.

It's easy enough for enforced laws to be avoided; really, all a teenager has to do in order to even gain access to alcohol is to just take it from mom and dad's alcohol cabinet. I'm not saying every minor would do this, but that's just an example of how accessible it is - marijuana will become the same.