This prompts many pop music listeners, like myself, to wonder, Where in the hell is Christina Aguilera? But then I remember something very important: Christina has a better voice than anyone, ever. (Except maybe Whitney Houston.)
Glee couldn't possibly tribute Christina Aguilera (or Whitney Houston) because there's no way they could muster the vocal adrenaline (see what I did there) to be able to sustain her kind of power for a whole episode.
It would be kind of like this video.
Here's Christina Aguilera's 'Not Myself', off of her latest release album, Bionic, which is really quite a good album despite almost universally bad sales. I'm developing a theory that it's actually a concept album in which Christina Aguilera is exploring the two halves of her personal identity. When you look at the Deluxe album, she starts out with 'Bionic,' a catchy, high energy pop song with heavy synthetic instrumentals, and then you end with 'I Am,' with is nothing but piano and Christina. This is the second song, and its themes can be contrasted with later songs like 'Vanity,' where she does the pop music equivalent of a Walt Whitman - "I am completely in love with myself." (Only in pop music, that's, "I turn myself on. Yeah, I'm vain. So what? So what?")
I find this video somewhat comical, but maybe that's how I'm supposed to see it. Maybe it's a message that, according to Bionic sales, failed to get through.