It seems like watching a Mixed Martial Arts match should teach you something about sparring or self defense... But whenever I watch one of these matches on TV it always ends up having two men laying on the floor while they grapple each other. This seem boring to watch. IMHO....

I'm wondering, does ground fighting in MMA result because of the rules? If we have to use TKD to defend ourselves on the street, are we likely to be at a disadvantage since TKD doesn't involve ground fighting?

well, thinking that we have to use what we learned on the street can get you in trouble.  we practice basic techniques over and over, so that we can do it well without thinking too much in whatever the situation may be.  we can still kick and punch while we are on our back and someone's about to jump on you.  also, we can still kick and punch as we are falling.  don't think we have to be on our feet to kick and have to have our fist by our sides to punch.  the way we practice our punch allows us to use quick snap in a short distance, and that can generate a lot of power even if you are punching their head when your hand is on their shoulder.

Watching is not a usefull as doing.

Many people do not want to practice on the ground.

I am sure your instructor would add it to your class if you asked!

Honestly Martial Artist should know some basic ground fighting skills.

I can't speak for the variety of flavors of TKD that there are out there but WTF traditionally does not have ground techniques or weapons for that matter. Most schools offer some sort of defense training on the ground but it has been imported from the Sabunim's past or background outside of TKD. Personally I'm not looking to arm bar or knee bar anyone where my life may hang in the balance. If I am forced into that situation I want to disarm and disable completely. A recent article in a magazine discussed with masters from various arts on whether or not training in the gym really prepares you for what is going to happen in the street. Across the board with few exceptions the answer was a resounding no. I thought one of the more interesting quotes came from a Karate Master "What can I teach a student that will prepare them for someone hitting them with a brick, or a trashcan, or a teapot because in reality this is what happens". I think almost as important as the training you have is the attitude I am going to commit to a no holds barred act of violence against this person or group of people and I'm not going to stop when they say they've had enough I am not going to stop at all until they cease to be a threat. That is a lot to ask of someone who trains in a civil environment.

BTW back to the original question, I enjoy MMA but find it hard to find a place for it in the TKD arena. I've had more luck with Muay Thai and boxing, it gets hard to beat a boxer's footwork.