All of these can probably be explained by history. I'm pretty sure that TEXT() was meant as a way to provide localisation. getglobal() is NOT defined in lua (and is more complex than what you show here). Its existence had a purpose when Blizzard did NOT provide _G.
The Blizzard code is old. A lot of it has been untouched for years. This is not because Blizzard coders are lazy or don't know what they are doing but because they are cautious.
It's easy to make fun of them, but it's a lot more difficult to maintain such a huge amount of code for so long without having bits and pieces of it looking strange years after it has been written (but still works).
It used to be used in all the StaticPopup stuff too. I think the only reason they still leave it around is because a lot of people used it when making their own StaticPopup's and many old examples still include it.
I wonder if setting the __call metamethod of _G and "getglobal = _G" would speed things up or if it's useless. (I don't know about the C implementation of metatables, and Im not home to test anything, just throwing thoughts against walls :p).
The <OnMouseWheel> one is most likely there to nil out some inherited functionality.
if ( MISTER_SPARKLE and MISTER_SPARKLE ~= 0 ) then
if ( UnitLevel("player") < 10 ) then
if ( PlayerTalentFrame_Toggle ) then
That would make sense. WoW has been in development for quite a while before being released (5 years if memory serves), and assuming scripting support has been added quite early in the game engine, they probably started with 4.0.