When you move upright, you open up your hips and that increases blood flow to your leg muscles. I normally ride with my hands on the hoods. But on a long climb, I'll move to the bar tops because aerodynamics is much less of a factor.

Another example that illustrates this is time trial bikes. Generally, they have a very steep seat tube angle with the bottom bracket moved aft. This enables the rider to have his or her hips less bent than they would be normally on a road bike, making the "aero" position more comfortable.

If you are sitting really upright on your bike - i.e. straight up - then it's a good idea to get a sprung saddle or suspension seatpost. Otherwise any bumps you ride over will have the majority of their shock being absorbed by your spine. And that could lead to problems over the long term.