Why is a Code Contracts tools version system-wide instead of project specific?

Question

Why are the Code Contracts MSBuild targets installed system-wide, in the sense that only one version of the tools can be installed/used at a time?

Wouldn't it be possible to have the Code Contracts installed separately, and have each project import the code contracts MSBuild targets on a project by project basis? This would make it possible to install different versions of the code contracts tools next to each other on the same system. It would allow people to work on multiple projects that each require a specific version of the code contracts tools... which is something we will need in the future.

Ruben