I think it's more useful to think of the number as a quantification of how much that advertising is worth: that's the amount of money operators are depending on (one way or the other) to keep providing what they're providing.
How you actually get it to them is a whole different question. They've talked about micropayments and subscription models and other things, but ads have the nice characteristic of requiring zero overhead for the viewer. There's nothing to install; you "pay" just by having it on your screen. Whether it's actually worth it to the advertiser is insanely difficult to say, but they are (at least for the moment) actually forking over the money.
Everybody would love a more precise system, where you pay for the page views that are of interest to you, but that shifts the burden from millions-of-site-operators to billions-of-viewers, and they're all incensed about having to "pay" for something they were previously getting for "free". People keep trying things, but it comes as no surprise to me that for a lot of side, throwing a few basic ads onto the page for pennies-per-thousand-impressions is the easiest way to monetize their effort, at least for the vast array of small sites.
Big sites (like Slashdot) can do better, because the economies of scale make it worth the overhead to try to get money from viewers, and maybe some day we'll get that packaged down to a point where other sites can get it. But since the total sum of money is pretty substantial, I think a lot of viewers will say, "I hate ads, but I hate paying even more."