This is not a new mechanic from MM3, its an application of rules interpretations for monster initiative that has been in 4E since the first DMG.
DMs can choose to have groups of monsters (usually of the same type) roll one initiative and act together on that number. Its up to DM interpretation whether this means you take all of each monsters actions in turn, or like you implied, take the groups move actions all together, then their standard actions all together.
This allows a DM to set up their monsters for flanking before the attacks, or keep them in formation for a defence bonus like the OP asked.
However, this is a table issue, and some players may call DMs out for allowing such tactics that the players cannot copy without Readied Actions. Above is the way I run it, because (1) its easier for me to keep track of, in terms of which monsters have moved and which have not, and (2) sometimes the monsters, especially minions, need a little tactical boost to be competitive.