IMO it depends why you race. If it's to finish and socialise then by all means carry one. If you race to win and stopping to pick out all the thorns and put in a tube will mean theres no chance of you winning then there is less point in taking one.
I suppose a tyre change will mean you'll still be able to cycle out, even if you don't come anywhere near the rest. I'm assuming seeing as your mountain bike racing though your pretty fit, and in a 40 mile race your never going to be anywhere you can't walk out, so not taking a tube isn't going to threaten your saftey.