Why TFS does not allow multiple collections to be connected with a single same Build Controller?

According to here and here TFS 2010 does not allow multiple project collections to use a same Build Controller. Why? I'm going to setup some another more build controller as virtual machines. But it seems somehow not practical. Because our company is going to have several project collections. Is it a good work-around? Is there any better work-around except that using a single project collection for all projects?

Thanks in Advance

There is a hack to use a single server as a build controller for multiple collections: Link

You should use a new collection when you want the isolation. Examples why you want it are:

• Security
• Handover
• Isolation for multiple customers