I disagree. It's a race so you have to weigh up the risks. I'm probably about as likely to smash my face into a tree at a Southern Enduro as I am faceplanting the tarmac at Herne Herne hill and I wouldn't dream of wearing a full face on the velodrome!
I don't think you always need speed. Slip a root and face first into a tree. If you have it, wear it I reckon. Better to wear it than regret it.