The fact that one in ten e-mail users are buying things advertised in spam continues to make it an attractive business, especially given that sending out huge amounts of spam costs very little.
An old PC World article suggests that "even a 0.0001 percent positive response rate constitutes a break-even point". So if you respond to spam, you've just created an incentive for criminals to spam a million other people. I say that makes you seriously morally culpable. Don't do it.
The only way spam is going to stop is if we remove the market for it. Each time someone responds to spam, they guarantee the rest of us will suffer more of it. That's just wrong. Given the huge costs imposed on society by even a few responsive individuals, this might justify going so far as to make it illegal to respond to spammed offers. Even if the law couldn't be enforced, it might provide people some extra reason to not respond to spam. At the very least the measure should help highlight the societal costs of spam-responding, and so discourage those who do so out of simple ignorance. What do you think?