And then they just put it in spam folder and what do you do with this information (They read it from this IP)? Sure, you know that email address is live and active, but what good is that info?
There's a million different uses. I could send an email to my friend and figure out if they're at home on their wifi or if they're out and using 3G. It's a general privacy issue.
Actually, they make the request to a unique URL which is tied to your email address and that particular "campaign", letting the sender know that you opened and read the email — and that you're probably a good person to send loads more spam to.
Regardless of how good spam filters are today, not opening the emails leaves you with less spam, and a lower chance of false positives/negatives in the long term.
