How were weights chosen in Runge-Kutta (4) method?
You can enter a known function that is the solution of a known differential equation, write down the Taylor series up to say 6 terms, and then substitute it into the Range-Kutta formula or any other formula. 

Once you calculate it, you will find that with the Runge-Kutta formula, you add up lots of different constants which magically add up to zero. If you change the constants to what you proposed, most likely these constants add up to something that isn't zero, introducing huge errors into your solutions.