'I have a theory about this... runners like marshall who are so dominate in the open field really don't pay that much attention to ball security. I think it's ok because the positives outweigh the negatives. he has to keep his body loose and agile to be able to shake defenders like he does...
he needs to be able to use both arms for balance and other reasons... I don't think he's all that worried about having an iron grip on the ball. more worried about using it as a tool to shake defenders.
Adrian peterson is similar in the open field, extremely hard to bring down, yet he also fumbles a lot. Same with Calvin Johnson, had 6 fumbles over 2 years, B.marsh has had 7 fumbles over a 2 year span before. I'm pretty sure it's the same with a lot of really good open field runners. I don't see any way around it either. I've even seen some pretty dominant open field runners CARRY THE BALL IN JUST THEIR HAND, not even under their arm, to be able to use that arm for balance etc.