Havoc load out should simply be a bi-product of what your army needs. Not got enough anti tank? Throw in the lascannons/missile launchers and have a squad of them sit on the back line blowing things up. Need anti infantry? Autocannons and heavy bolters. Want a close support unit for your front line troops? Give them 4 Plasma guns, flamers or meltas and shove them in a rhino.