I think the idea is that you get bored with the missions and go wandering. I just spend half of Saturday going on foot around the edge of the map, without any mission assigned. I scouted some outposts, found some wicked weapons (like M107 .50 long range riffle with the worst jamming habits ever encountered), cruised on rivers, run around rail track, cleared factories, blew up petrolchemical setup of sorts, burned baddies out of Fort, sniped some people in train depo. And because I had no reason to be going back and didn't have to use the same roads again just going around, on foot, seeing 50 sqkm without a single map load was the best fun since Far Cry. Seriously, if there was no set respawn (but instead transports of soldiers going around in trucks to repopulate) and your missions were vague "walk 50 miles north, oh you haven't found it, you prolly gotta go 50 miles west" like in original FC, we would have very, very good game on our hands.
One of the biggest downsides to the respawn system is that it's triggered not only by you leaving the area but also by save reloads if you are far away enough. Let me give you an example - I was sitting on a ledge of a mountain, sniping away people in train depot. I only had 12 bullets left, so had to make sure all my snipes were head shots. I shot gas bottle, which took care of 3 guys in the middle, then shot flamable barrel to set grass on fire and finish off two guys hidden behind fence. Pressed quick save. Snipped guy on the roof. Pressed save. Tried to shoot fella hiding behind some building materials but missed. I pressed quick load. WTF. Not only guy on the roof was now alive, but the gas bottle returned to its original position and flamable can were standing back by the fence as well. Now that is just unforgivable.