There are two things at play here.
1. It doesn't cost more to make these multi-core CPUs.
2. This is the new trend in computing.
With these two things at work single core CPUs have been phased out. The problem is, the OSs and applications haven't all jumped on to take advantage of these cores.
On your point about no one needing more than one CPU for the web, I would have to say you are wrong. First of all, as web technologies get better people will need more power. Especially when you consider the recent trend in web applications. Java, javascript, Web 2.0 technologies, etc. These things will get better and will need more power. How does this relate to single core computing? Well, it seems the chip makers smacked into a wall on the Ghz front. If we would have stayed with single core CPUs this capped speed, the technologies would have moved forward while we would be having a terrible experience. Hence, core computing was born.
So, depending on one's usage a single cpu may not be enough even for the web. And this will be even more true in the months and years to come.
One last point, you seem to be linking the offering of an eight-core Mac Pro with those who have lots of money. And in fact, the two are not related. The people who planned to buy a Mac Pro before the eight-core standard configuration update we going to get a quad for roughly the same price. Now, they get eight-cores instead and can save money by going to a quad. You seem to be suggesting that we have blown our money on an eight-core machine, I suppose we should all go back in time and pick up a Power Mac G5 1.6Ghz for $1999. That way, we won't be wasting our money on all these cores.
Ok ok, I have one more point. While single applications may not take full advantage of the cores, the big advantage to having four, eight, or sixteen is multitasking.