The issue with having the crew fight among themselves was to create a unity on the Enterprise and an us-against-them attitude in the plots. It's not that the characters could never have disagreements, but he didn't want it to become a soap opera where the conflict was caused internally on the ship. Instead, it came from aliens or others off-ship. However, he didn't want space pirates because that idea was too obvious and cheesy. He wanted to set the bar higher than that easy go-to villain.