You don't have to use one. Dirty knee never hurt anyone either. You can't sit on the roll however there's a real can of worms involved in the use of the long cone type described. Use the thinner end to kneel on and sit on the fatter end, I've seen it done in Portugal and mentioned it to the marshals as something to look out for. Anyway as in all sports, honesty and integrity are often neglected in an attempt to exploit the rules to gain an advantage. Personally I prefer the shorter lighter smooth foam roller used in aerobics, 12cm was a good all round diameter. Don't know any humans who would need 18cm!