Actually they can lori. A very common one is activated carbon which I have used in the past to clean up chlorinated solvent spills in low volume pumping environments. I also have a much smaller unit under the kitchen sink. Filters can hold onto charged particles as well (ions), but the ions tend to compete for the sites with the lower valance ions losing out. Example, Na+ vs Ca++.
This is true, but most filters marketed for backpacking, as far as I know, don't inherently include this as part of the system. It isn't difficult to add an in-line activated carbon filter.
I use a Steripen Journey and like it - but it can be a bit finicky. It doesn't like cold-cold batteries nor does it like water low in mineral content sometimes found at higher elevations (the terminals won't recognize that they are submerged, so the unit won't turn on the lamp - I believe the Opti addresses this); the water issue is an easy fix with a pinch of salt, drink mix, etc. If you're an an area with tapeworms, prefiltering is advised, as UV does not neutralize those eggs (but they are large and easily prefiltered).
I carry spare batteries, AM tabs as backup, and sometimes prefilter with a cotton handkerchief designated just for that purpose.
As suggested, if filtering for 3 people or more, a pump (heavier) or gravity system (ie Sawyer) may be a good choice.
A backup, for everyone, is prudent with any setup. Steripens can fail, and filters can become clogged (but sometimes easily backflushed in the field), can be damaged if dropped or frozen, etc, all depending on the particular system.
+1 on Cl02 tabs over iodine. Everyone should have their own.
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