I don't own one of them "big pits" your looking at, just a Bandera, but I have cooked on a couple (namely Gregs extended BYC, and Phils BYC and MOAB).
Some from a non owner standpoint, I think the vertical presents "options". I have seen it used to cook things in (ie more butts, ribs, etc) but have also seen it used to keep a big arse endless pot of chili hot while the main chamber is used to cook.
Also seen it used at Greg with like 12 racks in the vertical to take advantage of the slightly lower temps to make some of the finest jerky I've ever had. Then I saw it reloaded with 12 trays of peppers to smoke and make some of Marks pepper creations.
And at Phils, 6 dozen ABT's in there, then at both Greg and Phils, using it to make the cheesy bread (look under recipe module, appetizers, Phil's Cheese Bread)
So I think the vertical adds versatility, and "space" while your main chamber is loaded up. And you would probably need to pull out the top shelf in the main chamber to put a BCTurkey in there, which would lower main chamber capacity, but without a BYC in front of me, I'm only guessing that.
(BTW, when you cast a spinner bait in a kayak, don't you flip over?)
"Boots and Pants and Boots and Pants and Boots and Pants..." - Maxwell
Bandera, M-BGE, WSM, Gold Kettle, Kenmore Gasser, 36Qt Turkey Fryer, ECB, Dutch Oven