Do you mean that Sitefinity uses mainContent IDs for administration or for the predefined template sites we provide?
Sample template sites that come with Sitefinity (like Personal, Company and Community) use class names and IDs like header, main, footer, sidebar, etc. for layout. Anyway, they are meant to be used without making much effort to set the layout of the pages. Therefore, to avoid confusion we use the popular names.
For the public controls like News list, Single News, Blog, etc. (that are actually rendered on your public site) we use classes rather that IDs and we have put “sf_” prefix to html tags for almost all controls.
We don't have a list of the used class names and IDs in the administration because they should not cause any problems for public sites. The exemplary predefined template sites are supposed to be used with relatively little changes so in that case widespread layout names should be helpful rather than cause confusion.
the Telerik team