Once again, it has to be said that the Lefty sketches, while funny, probably weren't the most responsible things the show has ever put out there. With that obligatory caveat given, this sketch does a good job of illustrating how different Ernie's point of view is from Bert's (and vice versa), and it also gives Bert a chance to even the score a little.
The really weird thing about this sketch is that Lefty starts out the sketch with ears, and then halfway through the sketch they disappear. What makes it even weirder is that customarily he doesn't have ears, so I don't know why they suddenly decided to put some on.
In spite of everything that can be said against Lefty, I really like him; I just love the way he talks ("Riiiiight!"). He also did some sketches not involving Ernie, and the following is my favorite of them all:
I love Stan. He's the man.