Glasses are vital if you need them and carrying a spare set, or two if space and finances allow, is wise.

I know at least one person who keep a couple of sets of extended wear disposable soft contact lenses in their kit. I don't wear contacts and I'm not sure a set lasts but it is my understanding that he gets at least three days from a set.

Even if you have contacts on hand glasses would be good to have. Some conditions, like exposure to industrial, or riot control, chemicals make wearing contacts a hazard as they can hold the chemicals against he eye.