The intent of the DF is to find a group and at least attempt to complete a dungeon. If a group of 3 ques and gets a random thats fine, however clicking the accept button on the DF prompt, is an agreement to attempt to finish a dungeon as intended. If after 2 bosses the group of three says we dont need the last boss only to as planned before hand, to vote abandon with the 3/4 vote. They have now canceled a dungeon for someone else while avoiding the lockout timer them selves.
Regardless of other players job, they waited a time in que as well as the time in the dungeon and now have to que and wait again. If the person is a dps the time can be extensive. That is intentionally wasting the time of another player in a game that they have to pay to be in. That is harassment pure and simple and I for one will take the time to fill out a harassment form every time.