Scott, I didn't say the effects of weight training were short term, I said the advantage was short term. If you focus on one area then you're going to have to focus on other areas afterwards to even it out. Narrowly focused training results in a short term advantage in one area at a cost to development in other areas. You don't get something for nothing.
You don't think the sacrifice is a problem. Some people do. It depends on what the person wants; weights are not universally beneficial for Parkour practitioners.
Peter, one of the biggest myths in the Parkour community is that physically strengthening your body decreases the chance of injury when practising Parkour. It doesn't.
There are essentially two problems:
1) If the demands remain constant then improvement in physical ability will make you safer, but in Parkour the demands never remain constant. As you get stronger the demands increase. Parkour involves constantly pushing yourself, exploring your boundaries and trying to get past them.
Sure, a 3 foot drop to landing will be safer with strong legs than with weak legs, but if you have strong legs then you're going to be practising with a 5 foot drop, not a 3 foot drop. The physical danger comes from the physical challenge. If you're physically stronger then your training will be more physically dangerous.
The only way to stay safe is to not attempt things that you can't handle. It all comes down to judgement.
Mental strengthening, the ability to asses our abilities, to assess the situation, to resist outside pressures, to deal with fear, to be honest with ourselves; these are the things that keep us safe. These are the things that determine what we attempt.
2) You can't develop the ability to 'assess movement situations' separately like you can with 'physical strength'. You need to be involved in movement situations to learn about them and learn to deal with them. To learn to deal with physically challenging movement you need to practise physically challenging movement. You need to be working at your physical limit, meaning the demands of the movements you're using are just as near your limit as they would be if you were physically weaker.
The chances of making a mistake and pushing too hard are the same whether you're strong or weak, physically. The only difference is that if you're physically strong the consequences of making a mistake are greater, because the movement is more physically dangerous.
The safest way to learn how to practise safely is to learn it first, before you physically strengthen. Mental strength before physical strength.
There is no doubt that you can strengthen everything you need to without using weights. Just try and do the things you want to do. If you want to get better at small drops you practise small drops. Every living thing has been functioning and improving that way since life began.
The unproven method of improvement is one of separating training and living, training using abstract methods like lifting weights or even Parkour to some degree. On the evolutionary scale that's a very recent human invention that has been introduced to enable greater specialisation, and specialisation only works when things are good. Throughout the natural world, the specialists are the ones that die out earliest when there's a problem. Adaptability wins in the long run every time.
For some people who are already very unbalanced the only way to regain some kind of overall balance might be to train in an unbalanced way. That's essentially what Parkour is. It's unbalanced training designed to combat the existing imbalances in human society.
It seems to me that if people are physically weak then they will be mentally weak also, because you need the physical training to develop mental strength.
Gaz, we might as well get something useful out of this topic.