A few things here that I think most are missing:
1. Mailchimp is targeted (and priced for) the small business. Most small businesses aren't going to have lists more than a few thousand, which means most of Mailchimp's customers are going to be paying less than 50ish bucks a month.
2. If you've got thousands and thousands of emails to send, and are paying multiple thousands of dollars to mailchimp each month, you're probably just going to use mandrill and hire a dev to help you out there.
3. For those customers that are spending larger amounrd (500-1000 bucks a month?), they're going to marketing managers at midsize businesses who would never get IT love and can easily justify the purchase against conversions to sales.
4. Lastly, most people who use mailchimp send more than 1 email a month, therefore, most of the commentary surrounding the pricing divided by one send a month isn't really accurate.