The idea of a post-PC era has been around long enough now that most people think it's a given that sales of desktop and laptop computers are in a permanent, spiraling decline.

The belief is that we'll eventually reach a point where those dinosaurs of yesteryear will quietly vanish, except perhaps for some in quaint special applications. Reality, unfortunately, has a way of intruding on long-held beliefs.

Recently, I was chatting with my colleague at eWEEK, Chris Preimesberger, who told me of a discussion he recently had with veteran PC maker Michael Dell. The word from Dell is that sales of desktop and laptop computers are up. Chris says that this is because there are a lot of jobs that you simply can't do with a phone.

To illustrate that, Michael Dell asked, "When your kid goes off to college, are you going to give him only a cell phone?"

Taken tens of thousands of times over, and expanding the view beyond college to the workplace, it's easy to see why larger PC models are holding their own. There are simply some jobs, ranging from engineering to content creation, that need a larger form factor. While it's certainly possible to type a term paper or a book on a tablet or even a phone, the issue isn't about what's possible, it's about what's practical.

But in reality, there's more to it than just practicality. What's happened to desktop and laptop personal computers is that the tools are now vastly better. Designers have learned important lessons from the mobile world in the creation of easy-to-use applications and interface design. Hardware designers are now making computers where the physical form factor fits the use and the needs.

For example, desktop computers have morphed from the box-and-monitor model that was the de facto shape for years into one where the form more nearly fits the function.

While box-and-monitor designs still abound (I'm typing this column on one), there are versions where the box is tiny and is sometimes affixed to the back of the monitor or to the wall or the side of a desk. Other versions put the computer and monitor into a single package, often with a touch-screen, such as the Lenovo Horizon that I reviewed in December.

This variety of form factors exists in every type of desktop and laptop PC, whether it's made by Apple, Dell, HP or someone else. One of the problems with phones and tablets is that they're stuck with a basic form factor that needs to meet every need, something no device can do well.