Every corps uses straight leg if you only look at the downbeats, its the "and" counts where the definition of the leg changes.
As for straight leg jazz running, if you are taught it right, it is very comfortable. If you are taught it wrong, or march it wrong it can be painful. We were never encouraged at devils to try to stop in minus 1 at the end of a phrase if we were jazz running, trying to do stuff like that is what causes injury.