The basic assumption is that their are some things worth more then one beings soul.Even if your life is the most agonizing drudgery ever, it is less than an eyeblink compared to eternity. If you believe in heaven and hell (and given that we're talking about making deals with demons in this thread, that's a fair assumption), then why would you even consider making the deal?
Say the ability to save hundreds, or thousands, or millions of beings versus your one soul. Would that be a something worth eternity in hell?
Supernatural had their two leads go to hell (at different points). One did it because he couldn't leave without his brother, the other did it to prevent Lucifer from destroying all of humanity (at the worst). Clearly, a considerably larger stake at play then the first.