the tools already have some numbers but I find the problem lies in:
1) Not looking at the numbers frequently, unless email marketing is already your thing
2) Having everything in one place - if you're using multiple tools, you'd have to log into each one
3) Cleaning up the data - if you send emails using mandrill from code, like a lot of SaaS companies do, the data needs to be cleaned up first and similar emails merged.
4) Merging data from multiple sources - more in depth insights can be achieved once you add google analytics data, and even Intercom data
As email is the tool bringin your users back to your app, I think its worth looking at more seriously than what I think most of us do, especially when you don't have dedicated people to do it, or you're flying solo and don't have time to mess around.