The closed guard is one of the most common positions in MMA, but without an appreciation of the danger of strikes, it isn’t any better than any other position. To prevent power strikes you have to kill the hips and that means must control your opponent’s posture.
In grappling, where sweeps and submission attempts score points, it makes much more sense to play a very open game. There is very little downside to opening your guard, especially if you play a mobile game that’s hard to pass. MMA is totally different.
I have seen Jiu-jitsu fighters lose on the ground, in positions where they should be dominating. I mean how many times have you seen a Jiu-jitsu fighter finally get his opponent to the ground, and then instead of scoring the win, get pounded into a bloody pulp?