I was discussing this with my hairdresser recently. Basically she said it's a load of b u l l and pretty unhealthy to leave your hair unwashed for any length of time as you get abuild up of oils and dirt build up on the scalp and can end up with scalp problems because of it. You need to use soap (or a form of cleanser) on your body to keep it clean so why not your hair?
That said I don't use products at all in my hair - just blow dry and if I can be bothered, straighten.
I can't go more than 2 max 3 days without washing. And it's not only my hair that feels yuck, but I get more pimples on my neck and cheeks if my hair goes unwashed from my hair touching it.