I agree totally. Any precious time you have for training should be spent training. Other reasons I don't encourage weight lifting is:
1. the boxers I know are cut like statues and don't touch weights. They just train HARD, lots of heavy bag drills and lots of calisthenics, and they still have a hard time making weight despite the fact they have practically no fat and dehydrate for weigh ins.
2. It is very easy to injure yourself with weights. One of my trainers has phenomenal skill and is in awesome condition, but he can never fight again because his shoulders are perminently damaged from a weight lifting accident. Despite surgery he's not even supposed to be training and there's nothing that can be done. His shoulders pop out on a regular basis and the more they pop out, the looser they get.
3. I've never had a boxing and/or kickboxing couch tell me to do anything but train and calisthenics. Some are opposed to weights entirely.
4. I used to have a weight routine before I started training and have seen more results with just training than with weights. A huge difference. Not only do I have more defined muscles, but they are muscles that have purpose. You'de be very supprised now much muscle you build just doing heavy bag work.
You'll never get everyone to agree on this, so to each his own. But seriously, it takes a good deal of time to enforce a weight lifting routine, so why not just spend that time training? Boxers have some killer muscles and are cut to the bone, even without ever touching weights.