Quote: Andy Gilder "I can't see teams using two out of their eight interchanges to take off and then bring back their starting dummy halves, leaving them just six for the big men in the forwards. Particularly not in the height of summer.'"
Like SmokeyTA is getting at with his point, injuries need to be taken into account. Because of that you really do need to have a hooker/HB on your bench otherwise if your hooker limps off after 10 mins you can be screwed.
You'd just have to do it almost like RU, sub your hooker off after 50-60 mins and then he's done and doesn't come back on. Risky especially for the likes of a St Helens who wouldn't want the likes of Roby off the pitch at the end of a match but each club will have different views on that.