Because I get the feeling you won't believe anything I write. So I'd rather point you to a resource that will both answer your questions with more evidence than I could bring, as well as having the side benefit of you learning.
- Powerlifters and bodybuilders, like most people, will choose sports that cater most to their body types. It's more the specific muscle groups being worked on, genetics, and drugs, that decide the perceived huge variances in physique. Google the images of powerlifters who've cut weight or keep a low bodyfat percentage. Do they resemble anything to you?
- Strength is goal-specific. A football player will focus on exercises, diets, etc. that will help in football. Some sports/task may require stronger arms, some may require stronger legs, some both, and often to different extents. If you just mean general fitness, "purpose" still applies. How much strength doe a guy who sits in an office all day and never lifts anything heavier than paper stacks really need?
- High reps, low weight, will get you bigger, stronger muscles. Low reps, high weight, will get you bigger, stronger muscles. Bodybuilders tend to do the former, because it prioritizes muscle size. A boxer may prefer the latter, maximizing strength gains while minimizing size, to prevent changing weight classes. Both methods will eventually lead to notable muscle gains (progressive tension overload + necessary dietary elements) and neither will necessarily make you "slower". Slower at what, walking? Punching? Swimming? How, and why would you not be able to compensate via skill training?
- Weight turning into fat, what does that even mean? Muscle doesn't magically change into fat, at least not under normal circumstances. (Hell even the cruddy NY Times know that http://www.nytimes.com/2005/07/26/health/nutrition/26real.html?_r=0 ) And you can have big strong muscles and good cardio/endurance... just do both.
- Performance enhancing drugs. They can screw up your body over time, if abused. Probably why you see so many muscle heads in their old age look like crap. Very common in both pro competition and recreational gym use.
Here's an article on hypertrophy (rep range/weight): http://www.bodyrecomposition.com/muscle-gain/reps-per-set-for-optimal-growth.html Read the whole site, it'll back up what I've said. Great information, often debunking gym bro logic with science.
Edited by Kirosia - 10/21/12 at 6:56pm