This is one thing that I think Vern Gambetta gets right. His thing is basically that box jumps are fine and all cause you can get a max jump onto them with the impact of landing, work on pure explosiveness and whatever. But if you go much above 30", even for the best jumpers, you're just practicing/demonstrating core strength, hip mobility and balance on the landing. And there are much better ways to do all of those things.
ya i'm fine with box jumps used as an assistance exercise to vert, you can minimize landings from VERT-ADA-HEIGHT by landing on the box.. they are also good for hip flexors.. i'd say you can go above 30" though depending on your vert.. I mean if your vert is nasty high, you could use 42" boxes etc.. but the thing is, you have to land without forcing yourself into the most insane hip mobility contortionist possible.
take dallas robinson for example:
that stupid loser took down his training mix.. bleh.. anyway he was jumping onto 42" boxes, landing very decent in a half squat position.. his SVJ is 42".
as opposed to, crossfit box jumping "landing":
Poll