I think that would respond that Gracie Jiu Jitsu does contain strikes, like the pisao kick to set up takedowns, kidney kicks from the guard like Royce used in the early UFCs, and strikes to open up submissions from mount and back mount.
Gracies have done plenty of NHB and MMA fights, as the Gracie in Action tapes and UFCs 1-4 will attest to.
Grappling only allows a venue to refine those skills without the ensuing trauma, especially if you are talking bare knuckle fights. Of course, from that filter, you then have to be super vigilant about not engaging in behaviors that would be punished if striking was involved.
Rener and Ryron recently posted a pic on their instagram of training in the gi with mma style gloves on.
On the combatives DVDs, they say up front that if you want to fight MMA, you should train a striking art.
Rickson recently did a podcast where he said that although he trained striking, he never lost his jiu jitsu focus. He was never going into a fight trying to out strike someone. I think that is smart, to remember where your strengths are. I think it can be hard to work so hard developing a skill set, and then no go out and use it.