I think that's totally valid. As web applications grow you run more and more into the need for new features and the need for common solutions, things that Rails already give you. For example, when I wanted to try out jRuby for an application of mine, one point that made that impossible was the existing usage of the sqlite gem. Had I used activerecord I could have moved it over to the jdbc api with a simple added gem, Rails would have set me up for that solution. Or locales: The construction I added into my software basically re-implements how a Rails gem is used.

But that's also the value in it: After you constructed a big enough Sinatra application you understand why and how Rails is doing the things it does. I think that's very useful.