My understanding is that generally when the sprinkler system is set up, there is only one common wire shared among all sprinkler solenoids. Also, assuming the expansion board is placed near the main controller, even if you have two separate common wires, they would be fairly close so both can be inserted into the main controller’s COM terminal (there are two ports on the COM terminal).
If you place the expansion board far away from the main controller (and thus use your own extension cable), then I can understand the reason to carry the COM wire from the main controller to the expander. But we made a conscious decision to leave COM wire out of the extension cable, because COM wire carries 24VAC, thus a mis-wiring or plugging in extension cable incorrectly can cause instant damage to the circuit board. It’s safer to leave it out of the extension cable.
I was looking at my specific case were I have 8 zones in the front yard and 8 zones in the back yard. The front yard valves are all grouped next to each other. A single nine conductor cable runs from the valves to the controller. Same is true for the rear.
So I have two nine conductor cables going to the controller. One of the cables I had strip the casing further back so I could get a longer common wire. That common wire then had to run up to to the com terminal. Not a problem just did not look as pretty as it could have.
Using A typical 9 conductor cable as noted and three expansion boards (56 zones) would mean jamming 7 common wires into 2 com terminals, making a pigtail or adding remote com terminal block.