most(if not all) of the time you get porn in your mailbox it is because you have looked it up on your computer. i'm not saying that you personaly did, but maybe someone ( a friend or whoever) looked up a porn (accidentaly, or on purpose) with your computer.
i dont know anyone that recieves junk porn e-mails that has never looked up porn. although i know people who get a ton of these e-mails that i know have looked up porn. and no its not that they are subscribing to them, because most of them are teens and their parents would find out if they had subscribed. this is just that by going to their site and clicking 'enter' you are pretty much giving them permission to e-mail you.
on the non-porn side of things, i think e-mailing does help build your buisness, i know quite a few people, who if the subject of the e-mail is just right, they will open and possibly order the services. of couse most of us here wouldnt do that, but i do know quite a few people who have ordered things jsut from getting 'junk' mail. so i think buisnesses look at it and say, " hey, even if we are getting only X number of poeple to buy from us by sending e-mails, that is X ammount of clients we probably wouldnt have otherwise." so they do it. thats just my 2 cents on the matter