Grounding an AC power plug is not earthing. Impedance in
wire dictates that a connection from incoming utility wire to
earth ground must be short, direct, and independent. Short as
in 'less than 10 feet'. Wall receptacle is typically many
times too far away. Making matters worse, that safety ground
wire (ie romex wire) is bundled with other wires and has too
many splices.

Effective shunt mode protection cannot exist at the
appliance. Obviously - all but no earth ground connection
which is why plug-in protector manufacturers avoid this entire
subject. Protection that can be effective is already inside
the appliance. That series mode and galvanic isolation inside
appliances is not overwhelmed when a 'whole house' shunt mode
protector is properly earthed.

Appliances already contain effective 'point of use'
protection that is effective IF a destructive transient is
earthed at the service entrance. Inernal protection circuits
that don't require earthing at the appliance AND assume
earthing will be performed at the service entrance. No 'whole
house' protector means appliance internal protection may be
overwhelmed.

Again, that wall receptacle has all but no earth ground -
due to excessive wire impedance and other electrical factors.
So plug-in protector manufacturer simply avoids the entire
technical discussion. They don't even claim to provide
protection from a typically destructive transient.

Greg wrote:
Appliances already have effective 'point of use' protection.

How effective can that protection be if there is no grounding
conductor in the plug?