Simply speaking because it suck in terms of their use of some dodgy stuff they named “automated abuse prevention system, Omnivore”. To cut it short, they will disable any account, unless after the hard work of looking some reason to disable it, they find nothing, anyway it looks so. They worship something like Acceptable Use Policy, but looks like they themselves hadn’t read it.
I tried to setup account for my customer, some stuff which definitely doesn’t violate Mailchimp’s acceptable blah blah blah, but account was disabled and test emails not sent. Apart from that you can not use Mailchimp to send info about affiliate system, or how to make money online etc, which can make their service pretty useless in most situations. The problem is, that some day, during your business campaign, you can wake up with your email not delivered and account locked.
Recommended article (with real case example): http://mywifequitherjob.com/mailchimp-vs-aweber-a-comparison-of-two-email-marketing-providers/
The score is 1, because plugin wouldn’t let me to set 0.