Let me rephrase this, if you are a guy with normal to below normal physique, and you want to increase your weight quickly so you can fight in a higher weight class, weights can have a use. If you are a person satisfied or trying to lose weight and already have a semi athletic physicque, weights are stupid for several reasons.
1. Weights slow you down
2. Most of the greats do not do weight training.
3. Weights will not make you hit harder, stronger, nor knock a guy out more
4. Weights will give you a false sense of strength and put strain and pressure on your heartand cardio system
5. Boxing is a game of endurance, speed and skill. I'd rather be lean for the obvious reason it is easier to have better endurance with small muscle mass. You watch the decathalon and see all the big muscular guys who obviously lifting getting lapped by the smaller guys with smaller physicques. This is no different than boxing. Put a big strong guy with decent endurance vs a small guy who can run all night and the small guy will give you death by 1000 cuts.
6. By round 5 your arms with lots of mass are going to get real heavy.
7. You will be fighting an opponent who will now punch faster, move faster, hit harder, have better endurance and every conceiviable advantage because you decided to lift. At best if you carefully pick your opponents, you will be able to bang them up for a while, but if they can take punishment or hit back you lose.
8. Cassius Clay, rocky marcian, ezzard charles, jack dempsey,sam langford or Foreman or Frazier would kick any of the bums ass arroudn today.