We use a PLD/RDM in Dynamis and Einherjar to hold slept mobs while DDs pick them off one by one.
It's not that necessary, but it's a job people enjoy doing (myself included), and it lets the sleepers be a little slack, or focus on other things without worrying about mobs waking up and coming for them.
As for calling, people have used PLD/WAR for a long time, they're probably just used to it. We tend to use a WAR (Or NIN in Dynamis if anyone feels like being on NIN), since they accomplish the same task while dishing out a lot more damage.
PLD/WAR is honestly pretty horrible for anything endgame. If you're straight-tanking anything that needs a dedicated tank, normally PLD/NIN is going to be more effective, and if shadows are completely pointless you get much more damage mitigation AND hate tools out of /RDM.
That's the same reason /RDM, /BLM, or /DRK beats /WAR in a crowd control situation, you have a lot more spells to spam to keep hate on multiple mobs.