Proponents aside, here are eight reasons why tablet PCs haven't moved into the mainstream.

1. The price is too high.

Compared with a typical laptop, a tablet PC can run from US$200 to US$300 more, meaning most tablet PCs run from US$1,200 to US$1,800, depending on functionality, Fiering said.

Gartner predicts that the price differential will not come down in 2007 and might not come down in 2008, depending on the prices of screen digitizers, the technology behind the screen that turns a touch from a finger or a stylus or the electronic impulse of a special pen into data that can be stored.

Fiering said that digitizer prices have proved "remarkably stubborn" and have not dropped as components tend to do in the PC industry over time. Part of the problem is that suppliers have not geared up for massive production, held back by fears that the market might not do as well as predictions, Shim said.

Even with higher prices, tablet PCs have sold a little better in vertical markets than earlier predicted, said Roger Kay, an analyst at Endpoint Technologies Associates. In settings such as health care or manufacturing, tablet PC users have found using a pen or a finger to check boxes on a standardized form is easier, eliminating the need for the keyboard, he said.

The leading applications that benefit from a tablet PC are for clipboard replacement and handwritten annotation, Fiering said. Filling out forms for inventories, surveys or patient care has proved the most popular.

Also, insurance adjusters, contractors and students can use tablets to take notes. For example, nontext input for college chemistry or math majors who use symbols and formulas has been popular, Fiering said. Note-taking is also important in work situations. Salespeople and doctors, for example, must interact with another person, keeping eye contact while writing instead of typing on a keyboard.

Still, the price premium for tablet PCs is a real problem for further adoption and not helped by the fact that conventional laptop prices have come down in recent years, Shim said. When first introduced, tablet PCs cost as much as US$800 more than a laptop, compared with US$200 to US$300 today, which shows there has been some improvement, he noted.

2. Touch technology hasn't caught on.

Despite the advantages of handwriting and touch to some vertical market users, touch technologies with a finger or a special electronic pen used in tablet PCs have not caught on in the mainstream.

The main reason they haven't caught on is that the keyboard is already widely used and is growing more acceptable to use every day, said Ken Dulaney, another Gartner analyst. "Many people have learned to type out of school," he said. Text messaging and instant messaging only contribute to the willingness to use keys for input, several analysts said.

Also, touch technologies in early tablet PCs have not been that accurate, unless the application is forms-based with boxes to check, Shim said. Touch also has not been promoted by vendors.

"Touch is intuitive, but there's so much more that we could do with it, and not enough applications have been written to take advantage of touch," he added. "Typing is a well-accepted input system, and keyboards are here to stay. We seem to become more and more attached to them."