Well, all except for the part about having experience in dozens of rooms and multiple subwoofer systems but personal experience has shown me that agreed, yes, it's definitely beneficial.
One way it's beneficial, around which I don't see much if any conversation regarding this point, adding a second or third sub to a "weak" subwoofer system will raise the left-hand side a graph by (the assumption being, all subs are in phase), adding more energy to that end of the sonic spectrum. Doing this has the affect of lifting the graph, further extending the useful depth of a subwoofer system's room extension.
Now, in reverse of the above, if one starts with a strong subwoofer system and adds a weaker sub to a system, then all bets are off as every time the sound track extends beyond the capability of the weaker sub, the weaker sub will drag the graph down at what ever sonic location on the graph, the weaker sub fails to keep up. A sort of energy null if you will as opposed to a null created by intersecting sound waves.
(watched U-571 last night.........again.