It's like telemarketing. One call at dinnertime is an annoyance. Ten calls a day and you begin to think about an unlisted phone number. Unsolicited email fills up a mailer. Those who have a limited amount of email space may miss important emails, rejected because their mailer is full.
People become enraged.
Your account may be terminated by your service provider because of the complaints. Put in an 800 phone number and you will receive a ton of complaints, or long black faxes that can burn out the termal unit, or you could be targeted with mail bombs which are huge files attached to emails designed to lock up your mailer. You will find your company listed on Net Advertiser's Blacklists, publicly embarrassed and all of your products will be boycotted. Then there are those who get really nasty. Last February, it happened to Symantec.