Well first you can install an odd number of heaters easily. Plus it is easer to keep this type of manifold evenly sized on both sides. I see the first in last out install mostly in commercial installs and see all heaters working evenly. Where as the standard parallel install I see in residential homes, I have seen more failed water heaters due to, where one pipe leading into the tee is a hair longer than the pipe from the other heater, which causes the heater with the shorter piping to work harder.SewerRats
I don't particularily agree with BW's piping schematic. What is it about the design that you like? I have to wonder about the draw on a last out method.