I mark targets because not every dps class in this game is an AOE power house. In addition most pugs I have been in communicate hardly at all aside from maybe saying hello, and unless humans have developed a new sense of telepathy I like having some order and direction in my dungeon runs so I mark my desired kill order. Honestly the only time I would not mark is when I am with my FC in a full group because I can trust them to kill quickly and efficiently unlike many pugs.

I do not think it is such a terrible thing for pugs to ask for target marking, because they are probably asking so they can avoid the wrath of the Tank or Healer potentially chewing them out for pulling aggro and dying. Now in runs like CM or things of that nature I could understand the smaller need to mark but in other dungeons it is helpful.