No, no reason other than it's not been tested with any other version. My worry was if eventmachine got upgraded for some other application, it could break log_agent.

At which point the fix would be to push out a log_agent gem fixing the version, or uninstalling the newer eventmachine ?

My thinking was that it prevents any unexpected changes any version changes for log agent have to explicitly made.
Maybe changing eventmachine to ~> is the correct way to allow upgrades but avoid potential major breaking changes ?
The only reason i was so specific was because the gem is native, but i don't suppose it makes any difference if eventmachine is always present before logagent and no install attempt is made by rubygems.