Because it costs us less and we have better deliverability. Also, 1% of 50 million emails a month is a lot of undelivered mail.
Campaign Monitor isn't what you'd use for your apps, you want something like SendGrid.
SG is probably the most pleasant email-related experience I've ever had.
the discussion around managing email in-house vs. paying a 3rd party to do it for you, is of significant complexity, as is evidenced by the length of the original 37sig post and this comment thread.
i think it's important for each and every company to evaluate their own unique situation - the needs of their product, the resources at their disposal, the role email plays in their overall business model, their relative experience/expertise in email vs. other elements of the customer experience they are building, the maturity of the company/product, etc. it would be very unwise to make a decision on this type of matter, based solely (or predominately) on factors like "successful company X does it this way", or "successful company Y does it that way."
every company is different, and they often face this specific decision at different junctures in their life cycle.
the most useful lesson that can be gleaned from this conversation (which i've really enjoyed following), is this: as a business leader, entrepreneur, developer, etc, you have options! if you want to do it yourself, it's possible -- if 37signals can do it, so can you. but if you don't want to do it yourself, or aren't confident in your ability (for whatever reason), then there are several awesome companies out there to choose from, each of which has its own strengths and weaknesses, which puts you in the position to select the most ideal solution for your unique circumstances.
that's all :)
Obviously the blog post was only talking about how effective it is for 37signals, but I fear many young startups will misinterpret it and waste a ton of time rolling their own mass e-mail solution.
The list immediately after that of some of the headaches that getting e-mail delivered entails (monitoring and responding to blacklists, various configuration, feedback loops, etc.) is a very good argument for paying somebody to learn and handle the details. I don't see a convincing argument that getting 1% better delivery is worth spending time on instead of doing something else; indeed, they make the argument that improving validation and reporting on the app side is a much better use of time than fighting for that extra 1% on delivery.