I would like to use a lighter framework than Rails (Sinatra/Ramaze/Camping) but am concerned that by doing so I will not be able to use a lot of shared libraries that have been tailored to Rails in the form of plugins. Is this a major concern or are most of these plugins usable across different Ruby frameworks?
Are there any other potential disadvantages in using a Ruby framework other than Rails?
You can still use gems in all of the frameworks you mentioned, so a ton of stuff is reusable. Want to swap in a new ORM, no problems. Want a fancy shmacy syntax highlighting, no problems. Rails has been making a huge push to move away from the old plugin model to use gems exclusively.
If one of the other frameworks fits your needs better use it. Keep in mind that when it comes to documentation and samples rails has more.
If I was learning Ruby and wanted to try out a web framework I would probably go with Rails not because its better, but because its got much better tooling and documentation.