To put it more simply, the particle-attached bacteria do not die as quickly, if at all, with chlorine treatment. You really should remove the particles and the micro size critters attached to the particles, not just kill some of them and then drink them down. They don't just simply dissipate into thin air when you chlorinate them, they are still there, and the now dead "bodies" of the bacteria and viruses are going into your system. <img src="/forums/images/graemlins/frown.gif" alt="" />
After UV treatment, ceramic filtering, and chlorine dioxide treatment of the water, I then carry 32 ounces of my water in a clear plastic bottle on top of my pack so it's exposed to the sun's UV rays most of the day. I always carry two 32 ounce bottles of ready-to-drink water. One in the pack and one on top receiving continued UV treatment, and rotate them out as I treat a new water supply.
I think you'll find that almost all plastic blocks UV quite well - the SteriPen in fact depends on this in order to prevent burning you with UV light. So, the plastic bag in sunlight doesn't do anything other than warm the water and degrade the plastic a little bit faster.
Also, the SteriPen suffers from the same problems that chlorine does - bacteria attached to large particles can be shielded from light. It also has taken this into account as the chlorine dioxide products have, as long as the water isn't too cloudy.
Finally, the dead bacteria isn't going to hurt you. Viruses with disrupted genomes won't do much either. It is a bit creepy though. Just a bit of extra protein and carbohydrates though ;-) Bacteria that produces persistent toxins (such as botulism) aren't likely to be in our wild drinking sources.
I like my SteriPen and I use chlorine tablets as an alternative, just thought I might comment on this.