I'm honestly torn on this issue. My main is a paladin, rarely do I do enough damage to get enough credit to actually get an item if someone else is farming them. It also takes a while for shield lob to register as a "hit" and so on if I'm trying to tag things. When I got to farm something, I've always kept to a 2-3 mob camp (I don't take up the zone). Usually I have a goal in mind (relative to what I need to level xyz craft) of what I need to get.

If someone comes up and starts taking mobs without saying anything at all to me, I would see that as rude. Thankfully, no one has done that to me yet in game. I've had a few people tag one an move on (which I don't have an issue with) or people whisper me asking when I'll be done, and then I whisper them back as I leave.

On the other hand if someone is taking all the mobs because they can manage to kill them quickly and not sharing spawns, that's a bit of a jerk move too.