What it's really bad for is anything that doesn't like higher voltage... It doesn't hurt the alternator any... But if you decide to try it hook up a volt meter and watch the number climb, quickly.... I've seen upwards of 70 volts... If you're just doing a brief off & on to see if the charging system is working it's pretty low risk, nut leave it off for a minute & bad things can happen...
Back in the late 80's auto manufacturers were playing with windshield De-ice systems... Laminated conductive coatings on the glass & circuits to isolate the alternator & direct the power straight to the glass... An iced over windshield would be ice free in less than a minute.... But when the circuity failed that full voltage would destroy all sorts of stuff....