I think of two extremes:

I remember the people I really admire for their courage and dignity who would have been silent.

And then I think of the other extreme: an unpleasant colleague who has to answer everything, even what doesn't need a nasty answer, with a sarky quip. (Even those 419 scam emails sent by the 100s apparently get a dose of gall.)

Let the enemies stew in their own pots, curdle their own milk and ruin their own tea.
Most of the time, if you are silent the nastiness just redounds back onto the person who wrote it.    Sometimes it's damn hard though.