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"They rolled around each reaching for the other's throat" -Oliver Sells in The black feather.

Being on the ground is dangerous at the least, but not knowing what to do on the ground is much more dangerous. Some martial arts prefer to fight on the ground because the surface can be used to pin an opponent, facilitating technique application. However, rolling around at night in a dirty city alley with a possibly armed assailant is hardly high on my list of positions of advantage. There is glass, sharp debris, cat pee, and God knows what else down there, this without counting the biggest grappling threat of all... criminals prefer to work in packs, and the ground is always the wrong place to find out that your opponent is not alone.
All this aside, if you can't finish a fight in about 15 seconds, chances are that you will end up on the ground. You also might trip, or get pushed to the ground. Sometimes you might want to "take down" your opponent. However you got there, you emphasis should always be to get back on your feet. If you took your opponent down, and are working on the ground, always keep an eye open to your surroundings. Civilians have been known to interfere (with all the good intentions in the world), on the wrong side. A good buddy of mine, who is a cop, took a bat to the back while handcuffing a felon. A passerby thought the officer (in plain clothes) was mugging someone. It wasn't until the felon stood up, that he saw the handcuffs. He then proceeded to re-aprehend the criminal just in time to be cuffed himself by arriving officers. A word of caution, since martial artists tend to be very proactive, clearly know what is happening before becoming involved in somebody else's struggle.
Always remember: Your first option is always avoidance. It's hard to run from the ground.

Left: An arm bar applied on the ground.

The more tangled up you get while on the ground. The harder it is to get out if you need to move. This would not be a good position to be in when Fin's buddies show up.
Left: Fin applies a rear choke during training.

In grappling the environment is commonly used as a base to restrict an opponent's movement  prior to applying slightly modified joint locking and striking.
Left: An arm lock combined with a choke.

Locks, chokes, and strangulations are common in ground fighting. For proper training, strikes should be added to add realism.
Weight is a very important tool, used to limit the opponents ability to move.