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(it will also get very scuffed up on the USB pins)
I've been carrying a Yubikey around on my keyring for about 6 months now, and it is still in basically perfect condition. The only (very minor) scuffing on the USB pins is from where it's been pushed into a USB slot - just like you'd see on any USB device.
The funny thing is that for the first 2-3 months of carrying it, I didn't use it at all. In fact, the entire purpose was to confirm that they were robust enough to survive on a keyring before I made any investment of time or money to actually start using them as an auth device. Over 6 months in (and over 3 months since rolling them out to others in the company) any concerns I've had are long gone...