This concept has been argued on here before. Basically, the Marines don't trust anybody else to provide air support because everyone abandoned them in WWII, so they want their own stealthish fighter for denied airspace.
The whole line of reasoning on the F-35B is full of contradictions. The MEU will be hitting the beach and launching F-35s off of a Navy LHC, but for some reason the Navy forgot to send a carrier to take down the IADs first. Pretty unimaginable, especially since the LHC will need the regular contingent of cruisers and destroyers for protection against subs and stuff. Okay, let's assume the carrier broke down or sank enroute, and the Marines still need some jets to kill things with radars; that's why they're stealthish, right? Well, first of all, I'm saying stealth-ish because, since they have no internal gun, they'll be carrying a gun pod. They're also going to have a shit ton of external stores because there are only six of them on an LHC. Six fucking jets. Which means two flying, two turning/on alert, and two broken. So possibly only two jets ready to fight with, best case with no bombs or tanks, ten AMRAAMs amongst them. I'm no fighter guy, but I bet they would have a hard time dealing with, oh I don't know, ELEVEN airborne threats.
Well that's what the Navy and Air Force are for right? Then why do we need F-35Bs? Radar-guided pop up threats only? The whole thing is nuts. The Marines would be better off flying Hornets or F-35Cs off the big boat while flying as many Cobras as they want from the helicopter boat.
And of course the B-model has had enormous development costs while making the A and C-models fat and worse overall.