HUGE MISTAKE Closing GUARD in MMA?

A friend of mine was teasing me about closing guard in Mma
them i explained to him , that all depends of the situation in a particular
fight.
So immediately he started talking about Kron Gracie witch happen to
be one of the family who revolutionized Jiujitsu .
And i guess he had a point, but i believe kron Made a huge mistake and he
paid for it .