To me the biggest advantage would be that all you have to do is run a reamer into a 7 mag chamber and clean it up and you have an instant 7mm/375 Ruger! Do the same thing with an existing 264 win mag and you have a 6.5/375 Ruger. Now the 300 win mag presents a different problem in that the 300 win mag case is a little longer and might need to be set back
What would float my boat is a beltless 300/375 Ruger with the approx case capacity of the 300 Wtby in a 24" unbraked carry gun.
Don't know about dies though