I think that everyone deserves a chance to fight, regardless of whether it's a waste of time.
As long as they're doing something (kiting enemies, looting ammo, sitting in ambush, ect) I don't see how it's an intentional delay. There's definitely a distinction between someone that's being elusive and hard to kill, and someone that's been sitting in a haystack doing nothing for 5 minutes.
You might be annoyed that they're not rushing in to die right away, but ultimately it's their choice to make and you should respect that.