The problem with plastic is many plastics are great for generating static.
If talking about a plastic case
AND a plastic cap covering the USB connection, then, yes, the device is pretty well protected.
But clearly, the plastic case on
this thumb drive is useless when it comes to protecting the drive or the data inside if the user has a large build up of static in his or her body, then touches (or comes too close) to the exposed metal contacts of the USB connector.
This is not a given. Nor is it a given a metal case will prevent ESD damage in all scenarios.
First, for a metal case to effectively be a Faraday cage, the case must be "continuous" with no large gaps or holes. In an RF "screen room", for example, used for tuning and maintenance of radio transmitters and receivers, the size of the holes in the screen is important and are based on the expected wavelength of the frequencies used by those radios. A screen room built for HF (ham radios), would be ineffective at blocking RFI with UHF radios.
If the case is formed by panels riveted together, or held by friction (like many side panels in a computer case, there can be no paint where the panels meet and there MUST be an uninterrupted ground throughout.
And to be most effected, that case must be grounded to Earth ground.
One must also not forget the possible potentials static electricity can easily achieve. Depending on the materials used in our clothing, ambient humidity, etc., just by squirming in our chair, we can easily build up potentials of 20,000 or 25,000 volts
in our bodies. Yet some of the most ESD sensitive devices can be damaged by a charge as low as just 30 volts (as in thirty volts). The threshold of human awareness is around 3000 volts. This means we can easily destroy a sensitive devices from a static discharge without even realizing a discharge occurred.
As noted before, excessive static can be generated just by sliding something into and out of your pocket.
But you say, "
plastic is an insulator!" That is true. And plastic is an excellent insulator. But the insulating value is also dependent on the thickness of the insulator, and the voltage potentials it may encounter. And again - static can easily reach into the 10s of 1000s of volts.
Also, plastics can crack.
And then of course, can we really trust the quality of the design or manufacturing techniques of every thumb drive we see?
So yes, "ideally" you typically flash drive (and the data it contains) is effectively protected from static. But $h!t happens!
Personally, I would NEVER trust even the best, most reliable thumb drive for the safety of my data. I am amazed when I hear folks using them as their only backup device.