I like that argument, but then how do you explain the lack of a dismissal for 2 x cautionable offences when one is a sin-bin and the other isn't? Or for that matter, the fact we don't use a red card (the universal symbol for "you will take no further part in this game and cannot be substituted") if a player is sin binned twice and can then take no further part in the game and cannot be substituted?
I go back and forward on if sin bins were a good idea, but one thing I'm absolutely clear on is that the methods chosen for signalling them and for dealing with multiples are much more confusing than they needed to be.