If there is a mod you want and the mod is for 1.5.2 currently,you would have to remove all your 1.6.2 mods to put in the 1.5.2 mod,as well as downloading an earlier version of Forge. With multiple mods folders,this would make the system more organized (Ex.filing private modpacks) And using mods incompatible with the latest version. You would be able to specify what profile would use what mods in the options tab,access earlier versions of minecraft to use mods that are outdated (But you wouldn't be able to go anywhere before minecraft version 1.3.2,don't want to stress the developers out),and on top of all that,organize modpacks to your preference. If you implement this,thanks!

Profiles have a gameDir option for a reason.