I can appreciate your view - and in fact I think that a moderation team is being MOST effective when it appears to the general forum community at large that they don't seem to really be doing anything. But ultimately I disagree.
If you go back a bit - and look in particular for closed threads - you can see that there have been threads where the moderators have stepped in and either calmed things down or cut them off before they got out of hand. It didn't happen often, and as time progressed, the need to do it has happened less and less.
In every large forum in which I've participated (and thats not a small number of forums or themes), the most successful forums in my opinion have run like this. The moderators get involved very very rarely, and for the most part the community at large keeps the forum running on an even keel.
The thing about online forums - largely due in my opinion to the "screen width distance factor" - is that if left to their own devices without any intervention, they will degrade. One or two negative influences act not only directly, but also as a catalyst. Over time, the effect of one or two strongly negative types of posters corrupts the entire community - driving away some, causing others to sink in their level "just a little bit" in response to something, and then just a little more, and a little more.
The job of the moderators is, in my view, to catch those negative influences and contain them, or redirect them. In my experience, 8 or 9 times out of ten, someone with the potential to be extremely destructive to a community turns out, with a gentle nudge by a moderator at the right time, turns out to become one of the communities biggest advocates. Its almost an experiment in behavior modification... (yeah, I'm going thru training a puppy right now so its on my mind a lot).
Once the community gets running in the right direction, the mods need to do less and less because the community itself fosters the environment that it thrives upon.