Personally, I don't mind. Look at it this way. The enemy is not yours. It's a pack of data that anyone can attack. I might be annoyed, but I don't mind. Look at it this way. If someone " helps " with your enemy, you can defeat it twice as fast. That way, you can burn through two enemies in the time it takes you to beat one ( hypothetically ). Which gives you more chances of loot, more gold, etc.