Is it bad to use dry shampoo if you never want to wash your hair?

Water IsEssential to: clean. Shampoo, like other soaps/detergents, does not clean. They are emulsification agents which can coat groups of fat molecules at a submicroscopic scale, so that fat molecules (water rejects) will now mix (via the charged surrounding soap molecules) with water & wash away. This is also why physical agitation is essential; it breaks up fat, & other molecules, so the soap molecules can cover.