Hmmm....fairly easy question.
Why would I buy a $600 6800Ultra when I can buy a $400 6800GT and OC it to Ultra speeds?
Why would I buy a $180 3200+ XP when I can spend $80 on an XP-M and get 2.7GHz out of it, when the more than twice as expensive 3200+ XP only hits 2.2GHz? To answer the 'Why dont you overclock the new stuff', well because the new stuff is basically already pushed to it's limits in some instances...the 3200+ XP being one of them. The XP-M has much lower voltages, which results in less heat, which makes it extremely easy to get high speeds. You would probably need water cooling to get a 3200+ XP up to 2.7GHz assuming you could at all, whereas the XP-M can get to 3200+ XP speeds on stock cooling for the most part.
Also when it comes to RAM, you buy high performance for bragging rights, whereas I buy high performance RAM and overclock it do DDR500 from it's normal DDR400 and my machine will waste the floor with yours.
And buy 'you' I don't literally mean you, I use it in a generalized way.
As far as the warranty goes, this is why you buy OEM.
But even then, generally you can get away with RMA'ing something even if you fried it, just don't tell em what you did
I fried a stick of kingston RAM overclocking it, sent them an RMA request and said 'My RAM caused my machine to reboot all the time, I guess it's broke'
It's all about acting like a newb.
Point is, the real question isn't why would you overclock, it's why WOULDNT you overclock if you can.
You get a bigger indepth knowledge of the PC parts when you have to know exactly how they work.