I've been training BJJ for 4-5 months, exclusively nogi, and I never saw a point in using mouthguard as I perceive that strikes to the face rarely occur, and even if they do, their accidental nature would impede the force taken to the face to not justify wearing mouthguard.

Two weeks ago I was rolling with a person with simmiliar skill and experience level, I was on guard and was able to sweep him and attempt to pass his guard, successfully, as I was positioning my head for an arm triangle, he pushed my head down and tried to lift his knee to pull guard again but it smashed my mouth.

It fucking cut my upper lip and bloodied up my gum good, if the knee had gone a milimeter or two lower I may have some loose or broken tooth, I was still spitting blood the next sparring round, and when I got home I had to press ice against my mouth.

I had to explain to my boss and coworker two days later that I wasn't a victim of a criminal activity or domestic abuse, one and a half week later the scar still didn't fade completely, Since then I bought and always wore a Yokkao mouthguard when rolling.

TLDR: Wear a mouthguard.