The Blower Motor Resistor is, well, a resistor. It resists electricity and gets hot, resists more to produce a slow fan speed and less to produce a fast fan speed. The more electricity it resists, the hotter it gets. I've seen it written that when a Blower Motor gets old, it often draws more power than normal causing the Blower Motor Resistor to get more hot than normal, leading to premature resistor failure.
Learning this, I decided to replace my Blower Motor and Resistor at the same time. I also replaced the Blower Motor Relay which I also learned can become pitted and lose effectiveness closing the circuit through the contacts to provide power to the Blower Motor. I replaced all three items for less than $80 and haven't had any heater problems at all... in over a year since I made the repairs.
More:
http://www.automotiveforums.com/vbul...d.php?t=340871