But the prospect is pretty grim, to be on the ground with multiple attackers on you. Unlikely that ground fighting technique is going to rescue the day. But that is no reason NOT to learn ground fighting.
It can be useful in some cases though. The assistant instructor of the TKD club I go to managed to tie up a guy and use him as a shield on the ground to protect himself from the other attackers in his younger days. (He said he'd have been in serious trouble if it wasn't for his really big friend shouting at them and scaring them off though.) Hardly a perfect solution, but it did keep him in one piece...