Mark & Bob,
The blower motors are designed to be able to run constantly without
damage to the bearings or windings. Just ask any motor manufacturer.
I have been to many motor seminars & the main failure mode is HEAT. A
motor with restricted air flow will run hot and die an early death.
Starting causes some strain on motors but dirty or restricted filters
cause overheating which will damage motors. A blower motor should last
as long as the furnace if it has enough air flow to keep it cool. That
is if it runs constantly OR cycles.

Running the fan set to on will increase humidity when cooling by as
much as 15% in high humidity climates. This is because when the
compressor shuts off the water left on the coil will re-evaporate into
the air stream. Get a data logger to record humidity and try it both
ways. I did. A Lennox manual titled "Equipment Selection in Humid
Applications" came to the same conclusion.

My fan motor on my 2.5 ton heat pump indoor unit draws 2.2 amps at 240
volts. Just over 475 watts at 0.9 power factor. at my utility rates
that is about 4 cents an hour. That is just under $30.00 per month
when running 24/7.

I leave mine set to auto. If you have cheap electricity or a different
climate or both, feel free to set it to ON.

The advantages are it will even out temperatures and filter the air
better. The disadvantages are high summer humidity in many climates,
high electric bills and drafty feeling in the winter. The best time to
run the fan set to ON is spring & fall.

Stretch