an other question : why isn't it enabled by default ?
On older servers only the three SME Server repositories were enabled to keep control on updates. In the mean time development policies have changed and to minimize extra work and patching on packages available upstream that can be implemented without harm. They are to be updated from the CentOS repositories (as SME Server is based on CentOS).
You probably upgraded a server from a prier version of SME Server. I believe that as of SME 7.1 the Centos base and Centos updates are enabled by default as long as you installed it fresh, this can be done because packages that should not be upgraded from the CentOS repositories are now actively excluded per the yum configuration.
The developers have decided not to override choices made by individual system administrators in the past, and therefore the repositories are not forced to enabled by upgrades. System administrators however are advised to adjust the settings to reflect the FAQ entry I posted.