Ah, the old "fill in the blank" song, eh?
Personally, the problem with that one is that it started out as a smaller, running gag. Sort of like the random questions about cakes wearing hats and riding bikes... problem is they let that gag fly too long, it got stale, and now it's part of the "structure" that kids come to expect. They can't get rid of it if they wanted to.
Of course, I work that into the fact that Elmo's (as Tweety Bird puts it) only twee an' a haff years ode. If you ever notice little kids, they all do random stupid stuff like that. Especially when they sing (or shout in some cases).