What is the MultiCastActionBarFrame good for?

[Duugu]

Clearly it is used by shamans with totems.

But is there any other class where this bar pops up?

[Fizzlemizz]

Seems to be Shaman only,
From MultiCastActionBarFrame.lua.

local SLOT_EMPTY_TCOORDS = {
	[EARTH_TOTEM_SLOT] = {
		left	= 66 / 128,
		right	= 96 / 128,
		top		= 3 / 256,
		bottom	= 33 / 256,
	},
	[FIRE_TOTEM_SLOT] = {
		left	= 67 / 128,
		right	= 97 / 128,
		top		= 100 / 256,
		bottom	= 130 / 256,
	},
	[WATER_TOTEM_SLOT] = {
		left	= 39 / 128,
		right	= 69 / 128,
		top		= 209 / 256,
		bottom	= 239 / 256,
	},
	[AIR_TOTEM_SLOT] = {
		left	= 66 / 128,
		right	= 96 / 128,
		top		= 36 / 256,
		bottom	= 66 / 256,
	},
};
local SLOT_OVERLAY_TCOORDS = {
	[EARTH_TOTEM_SLOT] = {
		left	= 1 / 128,
		right	= 35 / 128,
		top		= 172 / 256,
		bottom	= 206 / 256,
	},
	[FIRE_TOTEM_SLOT] = {
		left	= 36 / 128,
		right	= 70 / 128,
		top		= 172 / 256,
		bottom	= 206 / 256,
	},
	[WATER_TOTEM_SLOT] = {
		left	= 1 / 128,
		right	= 35 / 128,
		top		= 207 / 256,
		bottom	= 240 / 256,
	},
	[AIR_TOTEM_SLOT] = {
		left	= 36 / 128,
		right	= 70 / 128,
		top		= 137 / 256,
		bottom	= 171 / 256,
	},
};

local FLYOUT_UP_BUTTON_TCOORDS = {
	["summon"] = {
		left	= 99 / 128,
		right	= 127 / 128,
		top		= 84 / 256,
		bottom	= 102 / 256,
	},
	[EARTH_TOTEM_SLOT] = {
		left	= 99 / 128,
		right	= 127 / 128,
		top		= 160 / 256,
		bottom	= 178 / 256,
	},
	[FIRE_TOTEM_SLOT] = {
		left	= 99 / 128,
		right	= 127 / 128,
		top		= 122 / 256,
		bottom	= 140 / 256,
	},
	[WATER_TOTEM_SLOT] = {
		left	= 99 / 128,
		right	= 127 / 128,
		top		= 199 / 256,
		bottom	= 217 / 256,
	},
	[AIR_TOTEM_SLOT] = {
		left	= 99 / 128,
		right	= 127 / 128,
		top		= 237 / 256,
		bottom	= 255 / 256,
	},
};
local FLYOUT_DOWN_BUTTON_TCOORDS = {
	["summon"] = {
		left	= 99 / 128,
		right	= 127 / 128,
		top		= 65 / 256,
		bottom	= 83 / 256,
	},
	[EARTH_TOTEM_SLOT] = {
		left	= 99 / 128,
		right	= 127 / 128,
		top		= 141 / 256,
		bottom	= 159 / 256,
	},
	[FIRE_TOTEM_SLOT] = {
		left	= 99 / 128,
		right	= 127 / 128,
		top		= 103 / 256,
		bottom	= 121 / 256,
	},
	[WATER_TOTEM_SLOT] = {
		left	= 99 / 128,
		right	= 127 / 128,
		top		= 180 / 256,
		bottom	= 198 / 256,
	},
	[AIR_TOTEM_SLOT] = {
		left	= 99 / 128,
		right	= 127 / 128,
		top		= 218 / 256,
		bottom	= 236 / 256,
	},
};
local FLYOUT_TOP_TCOORDS = {
	["summon"] = {
		left	= 33 / 128,
		right	= 65 / 128,
		top		= 1 / 256,
		bottom	= 23 / 256,
	},
	[EARTH_TOTEM_SLOT] = {
		left	= 0 / 128,
		right	= 32 / 128,
		top		= 46 / 256,
		bottom	= 68 / 256,
	},
	[FIRE_TOTEM_SLOT] = {
		left	= 33 / 128,
		right	= 65 / 128,
		top		= 46 / 256,
		bottom	= 68 / 256,
	},
	[WATER_TOTEM_SLOT] = {
		left	= 0 / 128,
		right	= 32 / 128,
		top		= 1 / 256,
		bottom	= 23 / 256,
	},
	[AIR_TOTEM_SLOT] = {
		left	= 0 / 128,
		right	= 32 / 128,
		top		= 91 / 256,
		bottom	= 113 / 256,
	},
};
local FLYOUT_MIDDLE_TCOORDS = {
	["summon"] = {
		left	= 33 / 128,
		right	= 65 / 128,
		top		= 23 / 256,
		bottom	= 43 / 256,
	},
	[EARTH_TOTEM_SLOT] = {
		left	= 0 / 128,
		right	= 32 / 128,
		top		= 68 / 256,
		bottom	= 88 / 256,
	},
	[FIRE_TOTEM_SLOT] = {
		left	= 33 / 128,
		right	= 65 / 128,
		top		= 68 / 256,
		bottom	= 88 / 256,
	},
	[WATER_TOTEM_SLOT] = {
		left	= 0 / 128,
		right	= 32 / 128,
		top		= 23 / 256,
		bottom	= 43 / 256,
	},
	[AIR_TOTEM_SLOT] = {
		left	= 0 / 128,
		right	= 32 / 128,
		top		= 113 / 256,
		bottom	= 133 / 256,
	},
};

Which will be texcoords for the different totems, "summon" presumably for the animation where required.

[Phanx]

It was used by shamans for totems in Wrath, but it was removed in Cataclysm with the totem redesign, so now it's not used for anything for anyone.

Basically it provided one button for each totem element, and you could click to open a flyout with all the totems of that element, and swap which one was on the bar. There was also a button for casting all 4 selected totems at the same time via Call of the Elements etc., with a flyout letting you swap between a total of 3 different totem sets, and a 6th button for Totemic Recall.

However, in Cataclysm totems were redesigned to be short-term situational buffs, removing all the long-term raid buff totems, so there's no longer any need to drop 4 totems at a time, and no need for swappable pre-defined totem sets, so this bar was removed. Blizzard also added the general "spell flyout" system, so shamans who liked flyouts for their totems can still have them, but other classes now have flyouts for some related abilities too.

The texture coordinates in the code you quoted were for the backdrop and border textures on each button. See:
http://wowprogramming.com/utils/artb...I-TotemBar.png

[Duugu]

God. What a relief. I was really afraid of having to handle this thing with my action bar project too.
Thank you for pointing this out and saving me valuable lifetime. It already was on my today's agenda. ;D