I think sinatra is best suited for micro-applications development (no big surprise, it's a micro-framework)
Sinatra provides you with a sufficient level of abstraction to build almost everything you want quickly. And what I like about sinatra is that the framework gives you tight control over what your app is actually doing, you can really "feel" what you are writing. So, I would say Sinatra is a subtly balanced framework.
I also think Sinatra is attractive for "people who likes writing Ruby" as said before. You start writing ruby class before implementing a sinatra interface. (good exemple of this kind of workflow here : http://dev-logger.blogspot.com/2009/01/ric-rac-roe-in-soup-of-technologies.html)
