Also worth noting that if you have a good kicker, then a 40-20 gets you off the rack and into a great attacking position at any point in the tackle count. And to reference the OP, a 40-20 wouldn't be nearly as effective if just earned you a PTB upfield rather than a scrum.
What would you prefer after a 40-20? A scrum which looks more like a glorified PTB anyway, or a proper PTB with an extra tackle?
Most of you seem to miss the point. After the PTB it's going to be tackle ZERO - so effectively you got 7 tackles instead of 6.