Thread safety in Rails is a total pain in the ass. So many libraries out there are not thread safe. I have worked with many large rails codebases and there always are gems that somehow work their way in with major threading issues. I think its a legitimate strategy to run rails apps single threaded and multi process. Not necessarily for all apps or workloads of course.
Multiprocess is more often than not better for latency anyway. Otherwise a big serialization or other cpu clog will block up the other, potentially quick, requests on the same process.
