My group rarely does the "focus fire" (or "piranha tactics" as we've heard it called) thing because if you try to dedicate all your effort to dealing with one of multiple baddies you put yourselves in an easier position (tactically speaking, not geographically) for your enemies to focus upon your team's weak-link. Instead, they divide their attentions among their enemies to encourage those enemies to likewise spread their offense around.
It's a lot easier to survive, and recover, when everyone in the party is a little bit down in hit points than when one or two characters are way down and everyone else is fine.
Our 4e game is more like what you describe here. The fighter and paladin, in particular, may often find themselves engaging particular foes to lock them down and soak their attacks, while I'm then using some other foe to try and get at the invoker/wizard, sorcerer or cleric/ranger.
I couldn't say with any confidence whether or not it ends up being mathematically more efficient, although I know my group can get through many encounters without an extended rest. (The game is now at level 30, and I doubt there have been many more than 30 extended rests over the course of the campaign.) But I think it makes the game more interesting, and helps highlight the different capabilities of the PCs.
A couple of examples I can recall: at around 11th level, the PCs were fighting Bane-ites in a village. While the fighter (and his entourage of dwarven minions) were fighting hobgoblins, the ranger was taking control of the hobgoblins' behemoth, the paladin was (for some reason I can't remember any longer - maybe a rescue?) charging into a burning building, and the sorcerer was tens of squares away fighting some bugbears.
An at 29th level, having invaded Orcus's palace in Thanatos, the sorcerer was fighting a marilith, the invoker/wizard was running away from the balor and getting pummelled by Orcus's wand, the fighter was dealing with a different demon, and the cleric/ranger was trying to take down Orcus's lich offsider. There was eventually a team-up on Orcus - the cleric/ranger stunned him and then the invoker/wizard dominated him - but that emerged reasonably organically rather than via a deliberate tactic of focused fire.
As well as the variety of foes, I think the terrain of the situation can help produce these sorts of non-focus-fire scenarios.