Marking mobs helps ps3-controller players.
Not only that, there are macros for auto-attacking num1 2 or 3 which helps dps deliver instant damage instead of "waiting" to see which mob everyone is attacking. It can also help the healer put down DoT's or sleep certain mobs (2 or 3 for example)

I don't normally bitch over mobs not being targeted. I use /assist <t> while targeting tank. But as a tank, I mark mobs always. It has shown me that it keeps greater order.
Just do it, there's no reason not to as you level.