I subscribe to DIR philosophy. However, some principles outlined above are a bit dated and could use a review. For example, what if you dive w/o a can light? Do you still need to account for its space? Lights have evolved and plenty people dive w/o cans in caves and wrecks. Also, if you want to be streamlined you may put the can elsewhere provided that you can operate the can safely (e.g., your controls aren't on the can itself but on a controller).
The scooter scenario outlined above is valid. But if you don't own the scooter, it makes less sense. You can do all of those things with sidemounted cylinders on both side.
The last point is quite interesting and troublesome at the same time. Before you jump in, you do gear matching so that you can understand your buddies' setup. I haven't been in a situation where understanding one's tank arrangement required more than a simple dialogue.
I have been sidemounting my bailouts since day one and found the setup to be extremely streamlined and easy to understand.