I'd be hesitant to leave it plugged-in for extended periods of time when unused, especially in a public place. I'm more worried about someone doing something silly with the electric cord, or some spike on the grid frying the bike's electronics, or the charger failing and causing a circuit breaker to trip and disrupt other users while the owner is away for 2 weeks. While I have not seen reports of fires with the Zeros while charging due to some failure, I've seen them for some other electric bikes. Why increase your chances of any of that happening if you don't have to??? There is simply no justification for risking these things when the bike is perfectly capable of spending a few weeks unplugged, given a sufficient initial charge. Other than convenience, I have not seen 1 reason that plugging the bike all the time is better than keeping it unplugged and charging only when needed (of course, proper care near low SoC and high SoC applies, and if what's that is unclear to the user, then by all means, keep it plugged in to be on the safe side). For those citing the user manual as the gospel - I'm pretty sure what's in there is adequate advice for the average Joe who does not need to be bothered to think about the bike's SoC or could forget it unplugged for months and kill the battery from over-discharging. Not necessarily the best advice for any one given situation though...
EDIT: I got some regulated power supplies at home, mainly for use with computers and other home electronics. They have relays to control spikes in the circuit that the filtering can't handle, so when they work I can hear them. I quite often hear them kick-in, so the grid power is less than perfect and could potentially damage sensitive electronics, including the Zero. I certainly would not want to have the Zero plugged-in during a lightning storm nearby. And keeping the bike plugged in at all times greatly increases the chance that it will be plugged in when the electricity cuts off or spikes back on during a storm, stressing the power supply on the bike unnecessarily...
Great. That guide explains really well why leaving the bike plugged in all the time might not be perfect in theory but is nothing to worry about.