freedom vs. responsibility?

This was originally posted on CouchSurfing but I thought I'd share it here as well:

Many of us have given up or minimized several things (eating meat, consuming, wasting electricity/water, driving, etc) to abandon the unethical, non-environmental way of living that seems to be considered as normal in the modern western society.
But we are still all travellers. I still dream of travels to the other side of the world, especially going to South America to volunteer on organic farms there. But I see the contradiction in that; polluting the planet and supporting airline companies just so I could volunteer for protecting environment?

How, and why, do you travel?
In what kind of cases do you justify flying (or any other kind of polluting transport)?
Are we, in spite of living in an "alternative" way, still seeing Easy&Cheap&Accessible travelling as something that we really need and deserve? Of course it's impossible to give up absolutely everything, but where do you set your limits?

The only morally acceptable ways of transport I can think of are walking, cycling, sailing and hitchhiking. I've met so so many people who agree with this but still end up flying (for example) because "there just wasn't any other way". What if the other way is staying at home, or travelling only where you can get by your own feet?

I also agree with everyone that there certainly is a need to travel in order to gain better understanding of life, the humankind, the world, or of yourself... This need as such isn't in question, but how about the way of fulfilling it? For most of us - depending on where you live of course - there are so many places (actually, more than you could see in one lifetime) reachable by overland travelling. Would these places be enough to fulfill this need? Or do we need the whole world to be available for us?
Seems like many people are still travelling to get more and more names into their list of visited countries... sure you gain experience in this way as well, but at what cost?
And which experience would be more enriching; travelling on a train / by hitchhiking to China through Russia, understanding the real distance of the journey and seeing all the places in between, or flying to Beijing for two weeks and then straight back home? By no question most would choose the first one - but then there are some problems like time. What if you only made a couple of such journeys in your life instead of making the two-week long every year? Would this be a less enriching and satisfying way, do you think you would gain less understanding or less adventures like this? Just wondering...