Simplicity's sake. On the 6.7's it's a little more tricky to run an o-ringed setup. Plus the 6.7s have proven themselves to hold up with the mls and 625's up to 1200hp with a well tuned matched setup. But that's just from what I've seen on all the research I have done.
saves time, money, and potentially adds a failure point due to a broken ring
plenty of people run the factory head gasket with studs to 60+ psi with no problems. Im sure youll get some people saying both ways are the best. Either way, do the proper torque sequence, the proper amount of re-torques, and get that motor to operating temp before you let the turbo sing and all should be fine