Usually, when I am swarming an opponent, I try to maneuver my units as far to the opposite as possible anyway, so I can swarm with the most units. Meanwhile I also have to take care so as not to expose my more fragile units, putting the tanks in the middle of the fight. I do not see how a flanking bonus would add anything new to this.
Oh, and I forgot to mention all the units with multi-attacks I have to position correctly...