I'm not an HAVC guy, but I did some extensive reading when I was purchasing an AC system for my house.
What I found is that anything more than MERV 8, can cause premature furnace or AC problems. Furnace filters were never meant to clean the air inside the house, they were designed to protect furnace components.
Furnaces are designed to have a certain temp. raise across the heat exchanger, or temp. drop across the A-coil and this range is fairly narrow. If the filter is restrictive enough, it may cause the temp. raise or drop to be outside of the allowable range. This can cause all sorts of things like cracked heat exchangers, weak AC performance, furnace shutting down prematurely, or short cycling, to protect itself when the heat exchanger gets too hot.
There are furnaces with variable speed blowers that will compensate for additional air restriction, but that uses more electricity and the units are very expensive anyway, so not a lot of people have them.
In essence, if you or someone in your family needs clean air, get a proper air filtration system.