Everyone has to start somewhere we arent all professional athletes. Using kettlebells will help you to get fitter, healthier and stronger.
Uh maybe because grappling/tackling is a big part of Rugby? Grappling is also highly dependant on upper body strength, some power lifters are surprisingly weak in some upper body movements. And how do you think Rugby players train? Deadlift, squat, bench, bench pull are their bread and butter in the weight room. Because of limb lengths some people are far better squatters than benchers, and vice versa. You really don't know what you're talking about, and you have the all or nothing mindset that stupid people have in common.
I think kettlebells can be a useful tool for boxing. A lot of kettlebell exercises involve explosive triple extension (i.e. extension at the ankle, knee and hips). When throwing a punch there is generally extension at the ankle (e.g. pushing off the floor with the right foot when throwing a right hand, and bringing the heel up). The knee will generally remained flexed (i.e. bent) but a powerful hip extension is an important constituent of a powerful punch. So, some of the movements from kettlebell swings are SPECIFIC to the movements used in boxing, and as such are a useful training tool for power development.
It's not proximal to distal sequencing, the knees and hips lock at the same, and the ankles have little involvement. Don't know many people who use them for less than ~6 reps at a time either.
So are you now saying they are ok for boxing? you know people who use them but for minimal reps! lol, caught! :good