I have never heard this as an argument against a plant based diet. Anyway, here area couple articles about it. http://www.ehow.com/list_7316466_foods-contain-cellulose_.html / http://www.raw-food-health.net/listofvegetables.html#axzz1SzuhDlzc / http://rawschool.com/best-raw-foods/
You could ask them why if it is so bad for you, many companies are adding it their foods now. Check this out. http://www.mayoclinic.com/health/cellulose/MY01762
I hope this helps you out a little. :)
The argument is valid. But there is a remedy to this: Eat more, juice your greens.