I use a dummy account when doing online business. If for no other reason to avoid clogging my real e-mail account with spam.
What I find is that there are certain businesses that just absolutely spam the heck out of that e-mail account. We're not talking shady sites or credit card companies - but the kind of company I have no issue doing business with. Except for their tendency to flood my inbox with junkmail.
Midway USA - Uhhhggg, the worst offender. I must get 5 e-mails a day from these guys, all "suggested purchases", "discount offers", etc
Amazon - second worst offender. Constantly asking me to "rate your transaction" or "review your purchase". I've actually tried to review a purchase twice, both times rejected because I wasn't singing with praise.
Nobody else floods my inbox, occasionally an e-mail will show up - but honestly once or twice a month getting an electronic flyer is nice. Keeps me up to date on what they have.