It does seem to cause a lot of confusion. Some places allow 310.15(B)(6) for most residential things, others do not. The argument I'd make for a garage or dwelling accessory building is what type of application do you think this is? Certainly, when you install a branch circuit in a garage or shed, you're not under commercial rules. No continuous lighting rules, no 180VA per receptacle. Many parts of NEC 220 spill dwelling rules over to attached/detached garages and accessory buildings on residential property.
The other part of 310.15(B)(6) that seems to cause questions is what is the main disconnect? If you have a 400A main disconnect, I would expect that you could run as many 100A or larger feeders out of that to various locations in a single dwelling and use table 310.15(B)(6) for each of them (as long as each panel was a lighting and appliance panelboard). I question how this is any different than having a 400A panel with 40 slots and feeders and branch circuits leaving it compared to the 400A disconnect that only has a single main breaker. To me, these feeders are still between the main disconnect and the L&A panelboards, there's just a lot of parallel overcurrent devices in the enclosure with the main disconnect.
