"Roger's articles say it doesn't matter if a drop or two's worth of untreated water gets into your drinking water."
Respectfully I'd take issue with that viewpoint. Studies have shown that in a single drop of water you have on average 10,000 bacteria (mostly harmless to us) up to 50,000 bacteria. Now if you're using a chemical treatment, then the drops will be killed, but with the steri pen, the treatment isn't persistent because the light isn't present. You certainly have a reduced risk because of simple numbers game (only a drop), but if that drop has e.coli or coliform and is introduced to your clean water, it will multiply rapidly.
All that said, the instance of becoming ill from these bacteria, protozoans etc is relatively low. The CDC (center for disease control) reports giardia as infection of 15 per 100,000 annually with one of the main affected groups being backpackers/campers/hikers.
CDC Giardia surveillance: http://www.cdc.gov/MMWR/PREVIEW/MMWRHTML/ss5401a2.htm
My thoughts are, I'd rather not spend 15 days on the toilet with drugs that will make you vomit often (treatment for giardia), so I am going to treat my water and if I am going to lug it there and back, I'm going to be sure to use it to it's full effect.
Just my two cents though.