I find rolling for "groups" to be rather unfair, because it ensures a bunch of similar (probably close together) monsters get to all act at the same time. By the time one orcish mauler or whatever acts, the players don't get to use that information on the other orcish maulers before they finish mauling for the turn. Spreading them out keeps the round fresh and fair, instead of "orc hits you with a sword and misses, other orc hits you with a sword and hits... 12 damage. other other orc hits you with a sword and misses" every round.
Also, to save "time" and bookkeeping at the table, pre-roll initiative. Very few cases the players can change monster initiative and if they can you can just change it with a pencil. If the number is going to be the same at the table anyway why not save the whole step? Note: there are some new solo monsters in 4es with pre-set multi-initiatives, so you can do that. I had a homebrew robot hand monster which had three initiatives and could do a given power once each per turn (one for each finger), that acted on 10, 15, 20.