I agree with Johnny here. It's very convenient to be able to pull in a newer version of a single package and have it work on top of an existing stable system. No need to debug MySQL 5 because I wanted PHP 5.
For individual packages this is the best route, and makes things work well for people who just want a more recent package or two. The down side is that this isn't always possible, and results in the repository itself acting in an inconsistent way. Some packages would be drop-in replacements like php or mysql5, while others are not. I guess it comes down to aiming for consistent behavior for packages in a repository, and finding out if the community is okay with a certain amount of disharmony in the repository.