However, if you're going to spend all day writing web-app code, learning rails pays off HUGELY. It's the full package for modern web-app development, with massive productivity bonuses at the expense of a higher upfront.
Sinatra, Cuba[1] etc. can finely be used for larger apps as well. It may require some more discipline upfront, but the notion that those are only usable for small hacks is moot. In fact, I have found it easier to introduce new coworkers (without prior knowledge of either) to medium sized (5k LOC) Sinatra apps than Rails apps the same size.
Everytime when you try to build bigger app with sinatra, you will probably end up building half-rails. So it the end it doesn't make much sense just the time it was needed to put in to find and require additional libs
Sinatra is great for small things and beginners.
However, if you're going to spend all day writing web-app code, learning rails pays off HUGELY. It's the full package for modern web-app development, with massive productivity bonuses at the expense of a higher upfront.