In many cases, doing that just slows the party down for no particular benefit. I was in a 4 man a while back (Haukke Manor) where the DPS were a level 32 (synced 31) ninja (myself) and a level 28 or 29 archer. At that level, NIN has no AoE attacks at all, and ARC has only Quick Nock (but usually without enough TP to cast it repeatedly). More mobs around wasn't more mobs taking damage, as we were only damaging one at a time regardless. More mobs in the group just meant that we ourselves would be taking more damage.
When I pointed this out, the tank replied that he did it that way because he didn't want the healer to get bored, but that's the most boring way for a healer to run content. It's far more exciting to keep swapping out between Cleric Stance and regular healing stance while balancing your damage with your heals to get the most out of both, than to have to just stand there spamming Cure - Cure - Cure - Cure - Cure - Cure - Cure - Cure. So, the overall effect was to combine making it more boring for the healer (nothing but Cure spam) with making it slower to complete (fewer players dealing damage).
Now, if it had been a higher level group and with Black Mages as the DPS, it would have been a different matter. Black Mages are good at AoE attacks and getting a big group of mobs together can speed things up by getting the most use out of each AoE they cast. But that's a pattern for that particular group makeup. It's not something that extends well to groups with primarily single-target damage dealers.
(To his credit, the tank involved was willing to listen to advice or requests, and did start taking mobs one group at a time after I pointed out the shortage of AoE abilities in our party makeup. Given how seldom I come across people who can take constructive criticism and adapt to it, I decided that was worth a commendation.)
No, I don't think that's an old school pattern. More like a pattern that's become popular recently because there are a few specific situations where it works, so some people who don't understand party makeup end up over-applying it.