Rugby Union

That's certainly the point of a 6-2 bench (set aside 7-1 as that was a slightly different situation). But I think its one of the things that is making Rugby more dangerous and counteracting any benefits to trying to reduce tackle heights and speed up the game etc.*

The idea of 6 forwards on the bench is you have your biggest players running at full speed for longer in a game, which means bigger collisions for longer. It also helps add to the dominance that defense has over attack in Rugby as well. The downside of that is that it leads to territory gains predominantly from referee decisions, when ideally referee's would be making fewer decisions.

On the old /r for Rugby there was a fairly constant debate between mostly Australasian/Pacific fans wanting to speed the game up, and SA/EU fans arguing that what they were asking for was to reduce two players from the field & just play League instead.

I think that ignores what Rugby Union is right now. The League criticism was 5-tackle, kick. But really Union has probably degenerated to 3-tackle, kick. Or worse, 3-tackle, penalty.

If we force forwards to play longer and not allow teams to substitute entire forward packs so they only play half a game, then the idea is that the forwards would have to boost their aerobic fitness and probably reduce size. That would (in theory) lower the impacts from collisions and eventually, through fatiguing the defense allow for the attack to find more holes and let the players make territory, rather than milking penalties and piggy-backing downfield.

*Though there is some argument as well that some players are more likely to make accidentally bad contacts due to fatigue.