The blower motor fails more often than one would expect, although 100K miles isn't all that terrible. Frankly, this motor looks pretty cheap, considering how difficult it is to change. It certainly is intended to be replaced when it fails, not rebuilt. Expect 100k to 125K miles of operation - no more than that unless you live in some heavenly clime that requires very little high speed operation of the blower for AC and heating. Heck, I live in moderately temperate Tennessee, and I have to use my blower on high a lot in both summer and winter.
Blower motor cost is between $100 and $150; installation labor alone might be double the cost of the blower motor. Other things that can happen (that may well be diagnosed as blower failures): Motor control circuitry can fail. And insulation can come loose from the AC air housing and get ingested into the fan. That makes the blower motor either stop or, more likely, vibrate horrendously and not put out useful air. The sudden load can also cause the blower controller transistor to short out (which makes the blower run on high speed - even with the ignition off - until you pull the fuse).
Symptoms: Blower motor doesn't run; blower motor squeaks (bad bearings); blower motor runs erratically. If the blower runs all the time, and only on high speed, then the blower controller transistor or resistor - not the blower motor - has failed. Excessive noise and vibration and poor air flow indicate insulation in the fan blower cage (which, by the way, can cause transistor failure in the controller).
Preventive maintenance: Possibly it would be worthwhile to remove the fan speed controller once every couple of years so you could reach into the AC air box and feel for loose insulation, although I doubt that you could reach the curved places where it tends to come loose. Absolutely check the security of the insulation in the condenser housing any time that you have the blower box removed. Glue it back down with a high quality contact cement after you've cleaned and dried everything as well as possible.
The most common failure of motor circuitry is resistor pack burnout (manual heating systems) and transistor failure (ACCII systems); the failure symptom usually is high speed operation only. Do not confuse this with blower failure. Repair parts cost ranges from $50 in the first case to $100 in the second; no more than half an hour of labor should be required in the first case, and four hours in the second case.
Insulation in the fan may well require removal of the blower assembly, although it has been reported that this problem can be fixed by pounding on the blower box to dislodge the insulation from the fan. Be advised that if you manage to knock the insulation loose, it will end up in the vent system, where it may or may not cause yet another problem.