While pvplogging is annoying and cowardly, I think it should be allowed because of the following reasons.
There are many different types of players in Minecraft, Builders, explorers, pvpers, greifers, and many other things. I feel like this allows the people who do not like pvp to avoid it.
I have teleported to safety many times before, some of those times from lava and environmental dangers, other times when I have been accosted without provocation. I have avoided MUCH distress and grief by doing this. Some people may call it cowardly, but if anything, it was cowardly for them to have attacked me from behind. I was only doing what I could to avoid personal annoyance.
As for those that attack others, then pvplog or teleport away when they realize they are losing, this IS cowardly. But, if they run away from combat with you, it is very likely that they will not attempt it again. So, why not be the bigger person and let the strife end there?
Lastly, there are a few times when a person attacks someone, starts losing, then pvplogs, then they continue the fight after coming back online, or teleporting back in to the fray. This is the only type of pvplogging I find offensive to the servers populous, and I believe this should be added to the rules as a reportable offence, resulting in warnings, kicks, or possibly as severe as Temp banning.
This is my opinion in this matter, thank you for reading.