As the title says, what are the effects of doing that? I know the electrodes turn a nice orange as they preheat, but what would happen if one pressed and held the preheat button (or say it got stuck in position-- would it kill the electrodes, and if so, after how long?)

It would have the same effect as a stuck starter. It would first deplete the emitter on the electrodes, and turn the ends black. Eventually, a cathode would break, interrupting the circuit. But then you have to consider the ballast, which will see above normal current, and run considerably hotter. Short/medium term, probably ok, but long term will take its toll on it.