for someone who has boxed as many characters as sam has (in any game) im sure hes aware of the difficulties and time involved.. having to res/repair targets and regroup after a while, for example. people that play single characters might say the same thing to us about playing 5 or 10. its all about perspective and what you enjoy. sam obviously enjoys pushing the envelope =P

as for tremor totems.. he's planning on starting after WoTLK. at which point totems will be raid-wide. 5 staggered tremor totems will completely eliminate fear within their radius.

micromanaging 5 shamans, 5 priests and 15 mages wont, i think, actually be all that difficult (aside from general boxing issues like lag, losing /follow, etc) as long as you keep things simple.

hotkey: castsequence/reset to cycle your totem field.
hotkeys: (example) alt+1 through 5. target groups 1 through 5.
hotkey: (G15, set to toggle on/off) spam r1 chain heal/CoH. obviously, this would have to have an in-game keybinding to trigger casting.. but switching heals on/off with a G15 allows you to start your heals and only have to switch targets depending on which group is being targeted, and theyll start getting healed. you can cast low rank CoH/chain heals all day long.. i cant see mana being an issue unless youre in combat for 10+ minutes straight, with no break to stop and drink.

then, your standard 1-9 keys are purely reserved for DPS/sheep/counterspell, etc with the mages.. which is exactly like playing a single mage, only multiplied by X.

so.. effectively, youre playing your entire healer setup with two buttons. target a group, toggle heals on/off. very simple.