I really liked using Sinatra until I used it with a team. The lack of ceremony and structure (as you put it) killed us. The app started out as just a simple service, so why not use Sinatra right? Then things changed and we were more focused on the web side, but stuck with Sinatra because everyone had heard the FUD about rails.

Each person has a preference on where they thought something should be. Days were spent arguing over the location of mundane things. Each team member had a different understanding of REST, so without the rails routing we argued about what we thought an ideal API would look like. We argued about which view engine we should introduce. We argued about how to handle our JS/CS. We argued some more about routes and how to make them more discoverable. On and on. Not intentional, but something new would come up and we would need to figure out where to put it.

This, more than anything, hammered home for me the strength of rails. Rails is great at getting you up and running, but it's greatest strength is that it's idioms are well documented. That gives developers a strong impression of how things are going to be and where things go. This saves you so much time by not having to argue and reach consensus on every little thing.