I get that, which is why the content is always geared toward being helpful, none of it tries to hard-sell the upgrade from free, except for the 50% off coupon.
It works because none of the emails are spammy like typical marketing emails.
If you were to just badger the users with hard sale after hard sale you would quickly have no users.
For me that was the smartest part of this system. I think anyone with a bit of humility wants a few tips to speed up their software adoption, login emails and coupons are always good. I personally hate those "personal" emails, but people are opening them so perhaps you are attracting users interested in your core philosophy if they know you are a startup, etc.
It works because none of the emails are spammy like typical marketing emails.
If you were to just badger the users with hard sale after hard sale you would quickly have no users.