There's some upside to having multiple register sets, though, and many quite successful ISAs have used them to good effect (CDC 6600, Cray-1/-2, &c.). It gives

There's some upside to having multiple register sets, though, and many quite successful ISAs have used them to good effect (CDC 6600, Cray-1/-2, &c.). It gives you more registers without having to use more bits for designators.