No one likes in when your chasing after someone and right when your about to land that final blow, They log out
I think a lot of people would agree this ruins the fun of pvp.
So here is a plugin to prevent it:
Combat tag
EDIT: I still cant post links so just go google: Bukkit combat tag
When someone does any damage to you you get "combat tagged" It stays this way till you go 10 seconds (the time is configurable) without getting hit by another player. Now your asking yourself, what happens if the player logs while "combat tagged". An npc of them is placed were they logged out with exact health and items. You can then kill the npc. (The npc will stand there for 10 seconds) Then the player logs into a death screen.