Why mark targets?

[Ultimatecalibur]

This is the standard response I get when I tell people I will not mark targets and they should not focus down one target at a time and that they should try to use their DoTs. I've even explained how the maximizing of DoTs is superior damage and makes for a faster, easier run. Unless my fundamental understanding of numbers is wrong, marking and focusing one at a time is worse DPS.

It may be worse outgoing dps, but it is better for incoming dps as dead targets can't deal damage.