Many kitchen items the area behind would not be acessable. Many things like ovens fridges etc are being installed much like dishwashers. Squeezed in, blind screwed into trims, and on occasion floors and countertops done after cabs can make them impossible to remove without damage. The maid aint pulling them out if I can't!
Why not behind a cabint back?
If you want to trick it out, get a matching scrap of cab matrial and velcro it on as a cover. Or epoxy it to the J-box cover. I have done both before for people who get antsy about wall plates. (With glass fronts) Otherwise I just use a 4-6 gang deep masonary box and a blank wall plate behind Lower cab drawers, where it will be least noticed.
As for conduit derating, you would have beef it up a bit for a kitchen IMO. #10's +... you might as well just run 12/3, or 12/2/2's (4 conductor) in regular runs. Either way, I do it often, and like the sort of central J-box idea for kitchens. Except for the single shot appliances like GD/DW...
