Yeah but it "helps" them by reading their email. So not only are those adds popping up in your messages, eating your bandwidth, are only from companies who paid to advertise in this method, but they completely invade personal privacy. And it might not just be the users of Gmail, what if you sent an email to somebody with a Gmail address and they quoted you in their reply. Is their email reading software going to stop at the new text, or does it read what was quoted as well?
I personaly don't need those ad's to tell me that there is some product out there that I emailed a friend about... If I really need to look up a store I'll search for it myself. And would appreciate them keeping their "helpfull" ads and spyware to themselves.