There's always someone who'll write that the discussion should be moved to il-talk, but there are many others who jump on the bandwagon and express their opinion or have some fun before the issue dies down. Invariably someone points a shaking finger at these last posts, telling the posters that they should know better, but everyone gets to kick the topic until it's finally declared dead for lack of interest. Simply put, it's boring talking to ourselves. We joined the list to express our opinions and hear others' opinions on a particular topic, but that's much too predictable and boring. We want and need the occasional incendiary post that wakes us up and gets the fun started.
People join lists in order to communicate with others.
Though before joining they probably assume that communication means a civil exchange of opinions, sooner or later they realize that communication means more than like-thinking minds congratulating each other on how perceptive they are. They discover that they need some outside input, something to spark some excitement. Spam fits the bill wonderfully. Spam consumes both time and resources. But e-mail would be so dull without it.