The demand for a G5 is all about staying State-of-the-art. The ibook is for the average consumer. The imac is for the consumer. The PowerMac is for the poweruser. The PowerBook is for the poweruser.
The PowerBook is all about performance. Right now the PowerBook lags behind the performance PC world in the following areas.
CPU performance, FSB speed, Memory speed and bus, Graphics card and Graphics Bus, display resolution and brightness, battery performance. The recent release of the updated PB's just aggrevated the situation by not improving the gap much.
When the TI 500 PowerBook was introduced 4 or 5 years ago. It was the most stunning laptop produced. It led the industy in CPU performance, size, weight, battery life, FSB, and had up-to-date graphics system, plus beautiful display.
As Apple fanatics, we just don't like being second place. Also remember hardware and OS improvements allow innovation in software development.
How many of you were saying, "I don't see why we need more than 32 mb of ram and a 68030 processor for word processing and spreadsheet work" back in the early 90's.
As new hardware and OS's progressed now we have software like iMovie, iDvd, Garageband, iPhoto, keynote, PhotoShopCS, etc, etc. All with functionality that you would not have dreamed of back in the early 90's.