I've never liked the practice of respawning enemies in any game, in any genre. It's a losing gameplay decision which takes away realism and puts the player in a handicapped position. Moreover, it takes from the player the feeling of achievement.
I can't tell you how disappointed I was when I returned to a previously "scouted" checkpoint, a territory I had considered to have "pacified" only to see my efforts were worth nothing and I had to go through a process having the *same* goal.
This is how you know a bad game design decision - if it makes the player try to achieve the same thing twice.
If anyone knows a way to disable enenmies respawning through the console, I'll be very thankful to know that. I'm sure there is some way to do it.