If you respond to them, you will get more spam. Even if they can't sell you anything, they can still make money selling your contact info at a premium price to other spammers, once you confirm your information is valid by clicking a link or requesting an unsubscribe.
If you choose to fight back like us, you're going to get more spam, not less. We're not doing it for our own benefit. But we've learned that the people that insist on sending unwanted spam in violation of various countries' laws are the same people that commit many other more serious internet crimes. The guy spamming you for illegal pharma websites isn't doing it from a computer that can be traced to him. He's using computers infected with malicious software that allows them to be controlled by criminals for profit. It might be your own mom's computer. It might be sending spam for Viagra one hour, participating in a denial of service attack to extort money from a business the next hour, stealing money from a charitable organization's bank account the next hour, and hosting someone's child porn collection on the side. One of the best known spammers is currently in prison in Russia for luring mentally handicapped orphans to his office and raping them.
It's up to you whether you want to be one of the ones who choose to fight them actively.