Basic OpAmp as a Comparator question
Is the primary reason to build a comparator using a voltage divider on the (-) input to set a threshold voltage (2.5 volts, in this case), preventing stray signals from triggering the output? (My intuition and very foggy memory says "Yes"). 


Yes, the primary reason would be to set a threshold voltage at the point where you wish to discriminate between high and low values.


  If so, is there any other reason to do so?


Yes. Your amplifier or comparator might not work very well with the input tied to one of the power rails. 

Top prevent "stray signals" or noise triggering the output a more effective method is to add hysteresis to the comparator threshold (or input) by using feedback.