I have a bit to add and I wonder if others would agree. Feel free to correct me if I make any false statements. Let's examine the main types of pathogens we could face in the backcountry:
Cryptosporidium: Filtered out by mechanical filters, not inactivated by iodine or most other chemicals unless you wait 4 hrs.
Giardia: Filtered out by mechanical filters, inactivated by iodine and other chemicals in 30 min.
Viruses: Not filtered out by mechanical filters, rapidly inactivated by iodine and other chemicals. I believe viruses are not considered a health threat in most parts of the remote US.
Bacteria: Most pathogenic types are filtered out by filters, generally inactivated by iodine and other chemicals. I believe bacteria are not considered a health threat in most parts of the remote US.
So, let's look at the overlap between filter systems and chemicals: basically, the only organism we're likely to face that both treatments get rid of is giardia. So if you've been switching between chemicals (including MIOX) and filters as I have, you've been sucking down whatever Cryptosporidium, viruses and certain bacteria are around! My point is that we may not have much to fear from these organisms in general.
So I'm calling on others to add to this discussion. First, do you agree with what I said above. Second, do you think Giardia is a big threat to our health? Many say that giardiasis generally does not come from wilderness water sources (see http://lomaprieta.sierraclub.org/pcs/articles/giardia.asp). And third, is it worth the risk of ingesting toxic chemicals or carrying heavy filters to protect us from a threat that is probably overstated?
To wrap this up, I'll just add that I don't trust water that has cow or horse turd floating in it under any circumstances.