Conventional wisdom about mobile computing says that end users are willing to carry only one device. This belief has led vendors to race to create the perfect single product. The problem with converged devices, though, is that they require compromises on functionality, and in fact the single-device notion is more myth than reality.

Based on a recent JupiterResearch consumer survey, we know that while users prefer to carry only one device when that is possible, they are actually willing to carry up to three, based on contextual circumstance. But there's more to the story than that; size is critical, and that's why it's important to break down the form factors for mobile devices into four categories. If you're making decisions about purchasing mobile technology for end users, you must keep these four categories in mind.

  • Devices that require an additional case.
    Any device that requires its own case, like a projector or large laptop computer, means end users must carry a significantly larger load, in terms of both bulk and weight. Because users must make a concerted effort to carry such a device, they will do so only when they need the dedicated functionality.

  • Devices that are cased with other devices.
    These are things that fit into a case that the user is already taking along. If a user is already carrying a bag that holds a laptop, taking several smaller items (such as a BlackBerry and cell phone) in the same bag requires little extra effort.

  • Pocketable devices.
    These devices are carried independently, on the person. There's a stark line of demarcation between this category and the two already discussed. A lot of things can go into a laptop case, but there are only so many items that can be carried on the person. As a rule, pocketable devices are worn on the person and are noticeable. As each device is added to the mix, bulk and weight grow significantly. As a result, our research tells us that most users will not carry more than three devices on their person, and two devices is the sweet spot.

  • Invisible devices.
    This is the most interesting category. Users do not hesitate to carry devices that they perceive as invisible. Watches, wallets and keys all fall into this category. ncreasingly, cell phones that are small and lightweight are being perceived by those who carry them as invisible as well.

What all this means is that vendors are racing in the wrong direction to meet a user need that isn't there. For example, reducing functionality in the interest of making a device smaller is foolish if the device isn't made pocketable. Likewise, increasing functionality while losing the ability to be carried ubiquitously can be wrong as well.

IT departments need to be careful when selecting devices for end users, and form and function need to go hand in hand. At the same time, users shouldn't try to sacrifice functionality for the sake of device size. Trying to replace your laptop with a BlackBerry or Treo might be feasible on a day trip, but if you're going for a week and need to update your five-year sales projections, take a real computer with you.

How many devices do you carry on your person and in your bag when you're on the road? In a future column, I'll publish an updated list of the most popular things people take with them and why.

Michael Gartenberg is a vice president and research director at JupiterResearch. This article appeared in Computerworld.