Ultimate exists for one reason only, and it's something that's tought in Economics 101.
If you sell your product at $100 you might sell 10mln copies. If you price it $50 you might sell 30mln copies instead, so obviously you make more money with the lower price. However, there's 10mln people who would've paid $100 who now only paid $50, so that's money you could've made but didn't.
The way around that is market segmentation: you charge more to the people who want to pay more. Of course, you can't do this too transparently because people will cry foul. The most common solution is to release the product at $100, then take the price down after a few months. That way the people who are willing to pay $100 will pay it, and people who aren't willing to pay $100 can buy it later.
The other option, which is what MS is doing, is to have different versions of the product. MS knows there are people who are willing to pay more just for the idea they have the "ultimate" version, and of course MS wants that money. The people who aren't willing to pay that much can get Home Premium.
With Vista it wasn't done correctly though. By not making the editions supersets of each other, they forced people who wanted the features of both Business and Home Premium to get Ultimate. This might sound good from a marketers perspective, but in reality it meant that a lot of people who weren't in the "willing to pay more" group had to buy Ultimate to get the features they wanted, and this created backlash. Windows 7 appears to have done away with this concept.
This isn't anything strange or unusual. Everyone does it. Airlines in particular have turned it into an art, to the point that everybody in the plane will have paid a different price for exactly the same product.