The so-new-it-isn't-in-service yet F35 fighter was supposed to come in three models, "A" standard for the Air Force, "B" Vertical/Short Takeoff and landing for the Marines, and "C" carrier equipped for the Navy. The B model is in trouble. Important pieces of the lift fan need to be redesigned, it's late, and it's getting expensive. The Pentagon (or someone) floated the idea of cancelling the B model to save money in Aviation Week.
The Marines wanted the B model, probably 'cause they could operate it off the big helicopter ships. These ships are built to carry and support Marine landing parties. They have a big flight deck for helicopter operations but it isn't big enough to fly fighters. The F-35B would be able to operate, giving the Marines fighter support flying off their own ships. Marine Corps institutional memory reaches back to WWII when the Navy would dump the Marines on a beach and then pull their precious aircraft carriers back out of danger, leaving the Marines to tough it out without Navy air support. The though of having Marine Corps fighters to support Marine infantry must be very appealing.
Actually, they could probably save money by merging the A models and the C models. The only real difference between a carrier fighter and a land based fighter is the carrier fighter needs a tail hook and must be more stoutly built to withstand the tail hook yank, the catapult heave, and the carrier deck whack. No reason the Air Force cannot fly the same plane. Granted, a special model that doesn't have to do carrier landings could have a little more range and payload, but the difference is small and the savings in having just ONE version of the fighter are huge.