The premise for Mystery Science Theater 3000 is quite brilliant find some of the most horrible films and have a group of spectators comment on it. The concept works because of the writing the writers of MST3K are made up of a motley group, whose various backgrounds lead to the show's cultural references that run the gamut from the arts and literature, sports, politics, and plain low-brow humor. The show is basically the end result of what would happen if you got together some of your most culturally-diverse friends and plopped them in front of a painfully awful movie with popcorn and, (let's face it because it can't hurt), a bottle or two of beer. This is what makes it likeable and easy to watch on multiple viewings.
The segments between the actual movie-watching verge from the ho-hum to the downright silly. I wouldn't necessarily say that they are essential to the show, but it doesn't hurt to get to know the characters who are enduring through the film with you. They were exiled in space and are being punished with bad movies, though this is not really needed in order to get the concept. Still, it does add to the fun of it all. If there were a cinematic equivalent to Chinese water torture, I suppose that this would be it.
Speaking of which, probably the only other questionable part of the show are the movies themselves. While the MST3K writers and actors do their best, there are unfortunately some movies they just shouldn't be attempted. These are the ones where you watch with the following sequence: Laugh, yawn, laugh, yawn, shrug at obscure cultural reference, laugh, yawn, yawn, yawn. This is not necessarily the fault of anyone involved. Some movies just cannot be saved.