Tubes do get dented by mis-use, eg clamping heavily in a bike stand, dropping tools, airline baggage handlers.
Dented frames are still rideable. If the frame is expensive and steel lugged or fillet brazed construction, it may be worth replacing the tube, but for welded construction, its simply not cost effective.
For every-day utility bikes, a thicker touring grade of metal may be preferable to ultra-thin metal.
Aluminium bikes are often made with fatter tubes, for increased stiffness. Manufacturers protect them against sidewall collapse by makeing the walls thicker. An aluminium frame will have more metal than an equivelent steel frame, but being 1/3 the density, they can still be made lighter, just not 1/3 the weight of the steel frame.