What TheLastCandle said. There aren't too many monsters that will cause insane amounts of trouble if they don't die first - it's just more efficient to have both your DPS attacking the same thing instead of the two of them whittling down two different targets.
If it's an AOE extravaganza or we're all so geared that everything's melting before us - marking doesn't really matter as much.
If it's an AOE extravaganza or we're all so geared that everything's melting before us - marking doesn't really matter as much.