All contributors have valid points IMHO.
None of the devices mentioned is guaranteed to survive a fall from height onto a hard surface- or other substantial trauma- without significant if not fatal damage.
That's equally true even for highly-engineered top-end smart-phones. Nevertheless some devices are inevitably by design more robust than others.
For hinged devices, repeated stressing of the hinge and its electrical connectors by opening and closing, particularly if unbalanced or forceful, or by flexing to-and-fro from walking, bending, squatting etc. when held in a tight pocket, may also eventually lead to failure.
It's also true that the likelihood of functional and cosmetic survival for all these devices can be prolonged by the use of a case.
But there's a trade-off. The more protective the case, the less convenient it's likely to be in terms of portability and useability of the device.
As every use case is different, only an individual can decide where the sweet-spot lies for them between armour-plated protection and no case at all. Or maybe varies, depending on circumstances.
Engineering the hinge mechanism naturally also involves trade-offs between robustness and other considerations including cost, opening mechanics and form-factor. Devices with the thickness of the Zaurus Cxxxx or Jornada 720 would for example be very difficult to market as a 'phone today.
Whether the engineers have made the right trade-offs for most use-cases only really becomes apparent some time after launch with feedback from experience of real-world use, whatever efforts are made to test robustness in the factory. Recall the bendy iPhone as an example. Evidently the Apple engineers didn't think to jam prototypes in their jeans back pocket and flop down forcibly onto firm seats.
Planet have clearly decided that the hinge mechanism could benefit from being upgraded cf. the Gemini. Despite this, it being an inherent weakness of clamshell devices, no doubt some will fail sooner or later- as did some of the Zaurus and Jornadas. How many and how soon is at this stage impossible to say- all we can do as individual owners is remember that these are not unbreakable devices and so look after and protect our own device as well as circumstances allow, or accept that ours might be one of the earlier ones to fail.
