IMHO, the switch rating is related to the connected load that the switch controls. Also, a good designer takes into consideration what type of "load" the switch controls. IMO, the resi guys use the 15 amp switches almost 100%. Comm guys tend to use the 20 amp rated. I can't remember having a call-back for a bad switch, as we install the 20's most of the time.
It's also a "spec" thing....if the prints/specs say "20 amp rated switches" then it's a no brainer.
John
