Sending out mass emails isn't as easy as it sounds. First it is difficult to draft up a professional looking email with good content. Unless you are a marketing major, this is something that every small business owner is going to struggle with.
Second, there are little tricks that are used to ensure that your email does not get filed in someone's spam folder. These can be found by googling, but it is likely that a SBO would not know them, and would not even realize that most of his/her customers are not receiving email at all.
Lastly, it is hard to track the success of your email campaign (unless you have really specialized tracking content as a part of the email).
Having an Email Campaign Manager that makes it easy for SBOs to send out emails about specials, discounts, promotions, new products etc would add a lot of value to QuickBooks.