We had the same suspicion but only one customer had observed it repeatably in the wild. We turned out to be customer number two. :) If you're running 1.9, definitely get agent 3.5 as soon as you can.
You're right, it's not that different - what killed us was unsupported gems that were wired deeply into our system, along with operations weaknesses that made changing ruby versions extremely challenging.
We took extra time to refactor those weaknesses out, so future changes like this one will be much simpler.
Incidentally, we don't parse log files - our agents send Thrift, JSON, and Ruby Marshaled data. String changes affected us a little bit, but the vast majority of the code in that tier is Java anyway. Only the front-end code is in Rails anymore.