My accesscab Tacoma is being built as a BOTT or bug out tactical truck. It's mission profile (worst case) is to pack 6-months of gear to survive in the field and be highly mobile.
Will I ever enter this type of mission profile, maybe not but I like to prepare my truck for worst case scenario's.
With that being said, the bed under a DCC Wedge topper is reserved for packing the 6-months of water/food, auxilirary fuel system along with other gear like hygiene, medicine totes and so forth... also gear for the truck, lubricants, etc... can go under the topper or on top on the roof system, spare tires, bikes on top of the topper, etc...
What would go on the roof system would be the tents, gear like shovels, picks that would rob space in the bed and other tarp's, etc... bulky stuff that can sit out in the elements while being mobile, tec...
The more space I can have in the bed under the topper, the more water/food and other stuff can be protected, etc...
Other than that on top of the cab becomes a dead space that does nothing, no value. With my front bumper, aerodynamics is already lost and being top heavy, well if you are a smart off-roader that should not be an issue. Ever seen how tall MTVR's are for the U.S. Marines?