If it does, request that the manufacturer provide you with the discharged MERV for the filter to determine its actual in-use performance" Also consider that high Merv rated filters have more pressure drop and reduce the air flow from a given system.Your point is well stated about the other even more important issues of fresh air ventilation and humidity control.The number is derived from a relatively complicated table that shows average efficiency by particle size within particle size ranges.
Keep in mind that visible particles start at 20 microns.
EPA Guidelines to follow as well: HEPA filters with a MERV between 17 and 19 are defined by the IEST test method as having a minimum efficiency between 99.97 percent and 99.999 percent in removing 0.3 m particles. HEPA filters have higher efficiencies for removing both larger and smaller airborne particles.
True HEPA filters normally are not installed in residential HVAC systems; installing a HEPA filter in an existing HVAC system would probably require professional modification of the system.
There are many things that contribute to the IAQ including air leakage of the building envelope and the HVAC system, humidity, system run time, indoor sources of contaminants, etc.. At the lower end of the efficiency spectrum a fiberglass panel filter may have a MERV of 4 or 5.
Upgrading a filter only changes one element and should not be looked at as a "cure-all." That being said I usually recommend a MERV 11 filter for those with respiratory issues. At the higher end, a MERV 14 filter is typically the filter of choice for critical areas of a hospital (to prevent transfer of bacteria and infectious diseases).