I do not give a sh*t about the company I work for, they would dump me tomorrow if they did not need me so I don't see why I should care about letting them down.
However, I work in a small team and we are all totally over worked so me walking out would have a real negative impact on the guys I work with.
In reality I do not know what I would do, the thing is, if I stayed, my heart would not be in it and the first "kicking" phone call I got from my boss over stuff I do not have time to do would prob result in me telling him what to do with his job and walking out.