supersix2

The part of the President's budget I agree with address things such as faster travel. I believe a large portion of that new funding will go to fund the VASIMIR project. NASA - Propulsion Systems of the Future

As far as simulating gravity, we can do that with centrifuges but keeping a spacecraft spinning fast enough to simulate gravity consumes a lot of energy and creates other control problems. It also changes the design of the interior of the spacecraft significantly since now planners have to account for the crew habitable areas along the outer walls of the craft. Working in micro-gravity has its advantages.

Due to the science and development we've done on the ISS, we've come up with medical solutions to reduce the bone and muscle degradation due to being in microgravity. We've also developed exercise techniques and equipment to counter the affects of microgravity on the body. Astronauts who return from their 6-month stays on the ISS have little or sometimes no significant bone or muscle loss. These are all things we never would have been able to prove unless we had a human presence in space.

As far as food production in space, the ISS has grown (albiet in small quantities due mainly due to the space available) various types of plants in space. Again this is a technique that would not have been possible to prove on the ground at all.

Also, my experience has shown me that even though hardware/techniques work on the ground, that doesn't guarantee it will work once you send it into space. The microgravity environment causes things to behave differently and sometimes it is tough to account for that when you are designing hardware. On that same note, since stuff has to be designed to work in microgravity it sometimes will not work on the ground since its design relies on the way fluids behave in microgravity. Also, a motor may be designed in such a way that it doesn't have the power to move a component on the ground because its not required to overcome the force of gravity once it's in space. The Space Shuttle payload bay doors are an example of this. They cannot open on their own when the Shuttle is on the ground.

Bottom line, if humanity wants to branch out into space and reach new destinations, every minute that we have humans in space operating in new and different environments we learn something. Even at the end of the Shuttle program, nearly 30 years after it began, we are learning how to operate the Shuttle better, safer, and in new ways that were never originally thought of.

Edit to address something I forgot:

To answer your question about our travel, yes it's true that our travel methods are based on gravity. Think of it this way. When we are in orbit around the earth, a spacecraft is essentially falling toward the earth and around the earth at the same time but since the surface of the earth is curved we never hit it unless we slow down the rate at which we are falling around the earth. So if you continue to accelerate, you will go to a higher and higher altitude but the earth is still pulling you back causing you to remain in orbit. Eventually you can move fast enough so that the earth cannot pull you back fast enough and you end up on a a parabolic or hyperbolic trajectory. You are moving away from earth but it's still trying to pull you back until you get further enough away that it's gravity is no longer strong enough to pull you back and eventually you would stop. If you stabilize there you've just reached a Lagrange point. If you are able to accelerate past that point using an engine, you'll eventually enter the gravity field of your target object. At this point, you are now falling towards that object and accelerating at it's gravity constant. If your energy is not correct, you will either accelerate yourself too fast for the object to capture you in an orbit, or you will slow down too much and crash into the object. Fortunatley for us, these equations to figure all this out have been around for a long time and when you are doing trajectory planning A LOT of people have to verify it.

You can completely circumvent this "free propulsion from gravity" if you have a lot of fuel and and a powerful engine and get there directly. Using gravity allows us to limit the amount of propellant and thrust we need. Using gravity, we only need enough fuel to simply get us to the correct speed so that gravity can do the rest of the work.