Plywood reinforcement on the inside face of the cripple-wall studs can save a house from destruction in a future quake. It's a cheap fix, and neglecting it is a bit foolhardy. Shear failure happens when the bottom of a building moves under the force of a quake but the top doesn't. Sliding failure - Sliding failure occurs when a house is not securely bolted to the foundation. Sliding failures are usually prevented by ordinary foundation bolts and framing anchors.
Many houses predating World War II, with deficient anchor bolts or none at all, are a worry.
Ground motion can send such houses sliding off their foundations. That's a pretty good argument for adding some more anchor bolts. Maybe and maybe not. Under the influence of earthquake forces, your foundation begins oscillating before your roof even starts to move. The result is that the top and bottom edges of your house's walls shear, or move horizontally past each other.