Why mark targets?

[Shinkyo]

snip

+1 on that one.
While Marking should only be a nice to have at endgame, it's still a good tanking practice. though it may not always add value, it's never hurt to do it.

As for the dots, it's not true always that dots yield superior DPS because of combo bonuses. it's really class specific.
Also, we're talking about trash pulls so applying dot on every target would only shortening the pull by a few seconds while increasing the incoming damage and targeting complexity.