I checked on the specs for your filter. The only thing I could find is that is would not be effective against viruses. That may be the reason for the advice that was given to you for the drops.
Below is a write up on the REI site that goes into more detail on understanding water treatment but one should note, "Viruses are the least common pathogens found in the wilderness. Viruses that afflict humans usually only reach backcountry water sources via human fecal matter."
http://www.rei.com/expertadvice/articles/water+treatment.html
http://cascadedesigns.com/msr/water-treatment-and-hydration/expedition-water-treatment-and-hydration/miniworks-ex-microfilter/product
MSR miniworks ex filter
Effective against protozoa, Yes
Effective against bacteria, Yes
Effective against viruses, No
Effective against particulate, Yes
Effective against chemicals/toxins, Yes
Weight, 1 lb / 456 g
Width, 2.75 in / 7 cm
Length, 7.5 in / 19 cm
Filter media Ceramic Plus Carbon
Filter pore size 0.2 microns
Flow (L/min) 1 liters per min
Flow (strokes per liter) 85
Cartridge life~2000 liters
Field cleanable, Yes
Field maintainable, Yes
Country of Origin Made in Seattle, USA
Mechanical advantage, Yes
Water Bottle Adapter, Yes
Cartridge replacement indicator, Yes