I think they're just trying to get people thinking about upgrading early. Supporting older software is really no fun so they probably want to drop Cent 5 support as soon as it goes EOL and not have people complaining.
Also 6+ support several new things like SNI for example that they're building around pretty heavily, as well as dropped Tomcat support, etc.
11.50 is still pretty new but to date I've not heard of any issues with it simply because of CentOS 5.