From a street self-defense training standpoint, it's good to practice defenses on the ground against
(That last is by far the hardest situation to deal with.)
- kicks from a standing opponent
- punches from a standing opponent
- ground-n-pound from an opponent who is on the ground on top of you
- grappling from an opponent who is on top of you
- weapons attacks from an opponent who is over you (either standing or not)
Yesterday afternoon a friend brought some training knives to our workout, so we spent a couple of hours sparring and training grappling scenarios (standing and on the ground) where one combatant had a knife and the other was unarmed. Even though it was a much more dangerous situation than working against an unarmed opponent the normal concepts still applied. It just added a new variable that had to be kept track of. Some techniques had to be modified, but the principles did not change.