My guild wanted to do this as well but I couldn't for the life of me understand the reasoning behind it. At the start of the fight you have 5 mobs on you and 4 mobs CC'd. Once one mob goes down you have 2 mobs CC'd and 4 mobs on you. Once a second mob goes down you have 0 mobs CC'd and 3 mobs on you.
At no point does the loss in CC cause you to have more mobs on you than you ever did previously. Levelling their hp out to drop at the same time just extends the amount of time that you have five mobs attacking you and also requires un-needed target swapping as some groups of mobs are going to get taken lower by cleaves than others (1 mender + 2 of each other mob in group = Trappers and Blademaster hp drops significantly faster than the mender group)
Anyway, the fight's easy enough that it doesn't really matter, I just can't really figure out what possible advantage you achieve by dropping the adds all at the same time.