For years, all of my pcbs have used simple series-R protection on the I/O pins, and
no one ever blew up one of my boards [at least that owned up to it]. OTOH, back
in the days of yore, another guy was selling similar boards with no protection and
all smt parts, and every single day on the associated forum, some poor guy came
on crying he had blown his xxxx board. Some designs are "more" idiot-proof than
others, I guess.
No finer example of design for protection then the Ruggeduino. There is a lot one can learn
from reading it's schematic.