One reason for leaving them on is if there's a major problem with the truck or camper. For example, the camper falls out of the truck and you need the jacks to get it back upright so it can be towed away as shown in this post: http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27461157.cfm
Pretty sure the odds of ripping a rear jack off on a tight trail are higher than the camper falling out of the back especially with a pop-up.