This would enable the mains to be proccess with external plugins, the same way all the inputs can be.
While this is possible by routing the main return to a pair of unused inputs, then routing that to an aux. This is not practical at all since it takes up an aux mixes and physical mix outputs since you can't reroute the main outputs rendering them useless. This would be great not only for the mains but also all of the mixes, more advanced users would really appreciate this.