The marginal costs may be low, but overall the value is in their ease-of-use, reports and analytics, and time saved in not worrying about email deliverability.

I think it depends on what features you need and how much time you want to spend on it. I recently set up their RSS to email feature, which is really cool. When I write a blog post that I want to be sent in an email campaign, I just give it a certain tag ("newsletter") and then the next day it automatically goes out to my list.