I will go out on a limb here and give my newbie working conclusion, which is ***subject to change*** as new information comes available. I think the recommendation for 80% is for the bulk of drivers out there who are driving far less than 70 miles a day. If you commute requires 100% charging, then do it, don't sweat it. The thing to avoid, IMHO, is charging to 100% each time you plug in, if you are plugging in multiple times a day for short opportunity charging to avoid dramatically increasing the number of 100% charges.
If I charged to 100% every time I plugged in, that wouldn't just be once a day, it would be up to 5 times a day and that is the important thing to avoid, IMHO. Charging multiple times a day makes 80% charging more than adequate for the vast majority of my driving and keeps the car charged up enough all the time for me to be comfortably spontaneous.
Gasless: Silver 2012 SL, traded in for Lease on 1/13
Tesla S P85, Gray, pano, carbon fiber, took delivery: 2-9-13... LOVE this car!
9.8 kW PV Solar installed 9/12, http://www.westseattlenaturalenergy.com