Basically, no-gi means using your opponents limbs and body for holds and chokes. A must-do training method for MMA comp.
Gi training is good for holds and chokes using the opponents clothing. Good for streetfighting and requires some ingenuity to translate to comp.
Bottom line...If you're going to compete in a ruleset (MMA, etc.), you must be able to work without a gi, one way or another.
Sapere aude, Justin.
The map is not the Terrain.
"Wheather you believe you can, or you believe you can't...You're right." - Henry Ford