If you're grouping with random people you don't know I'd encourage using it. For packs of two I only mark one mob, packs of three 1 and 2. Sometimes I'll mark a 3rd in 4 packs but usually by that point in the encounter AoE damage and such will have them ticked down and I will have substantiated enough threat that it really doesn't matter what happens after the first two mobs are dead. Smart DPS will usually migrate to the one with the least amount of HP for any unmarked mobs.

Now if only I could solve that mystery of why Archers/Bards think the mob marked 2 is exclusively for them...