Of course there are situations (certain down and distance or even more likely field position-goal to go) that don't call automatically for nickel.
But using nickel to combat 3x1 in typical situations was a lesson finally learned in the NFL by the last hold out Broncos when the 49ers tore their simple 4-3 Cover 2 to shreds in the Super Bowl which yielded a 55 to Doesn't Matter final score. John Taylor said at the time, this offense (both the original Walsh and the Holmgren 3x1 variation) was designed to pull apart the kinds of coverages you get from a base D, especially a Cover 2.
You have 7 in coverage assuming no blitz and almost always one (if not two) is deep safety. That's 6 to cover 4; you can double 2 which leaves 2 in single coverage. And those single coverages can be manipulated to put a LB on a WR or TE at will. See the Packers opening game against the 49ers when Perry was in coverage on Michael Crabtree.
You can play base or you can blitz in your nickel, but if its a medium to short yardage situation, you are playing with fire since the offense is happy with a quick throw.