I think of air flowing through the cook chamber as if its a river. And on the stack end, the river is hitting a dam.
If the cooker does not have the collector, that would be like having flood gates open in the middle of the dam. Flotsam and debris will accumulate in the corners of the dam. And it will create eddys.
With the collector , there's no build up of " flotsam " in the corners , making the entire grate usable.
I beginning to arrive at the conclusion, that Franklin's concern for air flow is on those 1,000 gal , 14 ft long propane tank smokers. I would guess that getting good draw through something that large is a problem. But I don't think small backyard cookers have a big problem with air flow.
Just my thoughts.