When you're missing a piece of the trinity you should feel like you're missing it.
This is true of tanks, their passive and active mitigation goes really far and they're increasingly self sustaining murder balls, something exceedingly valuable for any dungeon run.
This is true of DPS, you're not getting anywhere quickly without at least one competent DPS, we've all had those runs where both are asleep at the wheel and a trash pack takes two minutes.
This simply isn't true of healers, every tank except DRK can spot heal and many DPS have both self and party heals to pick up the slack.