I still do not get it. How often do you user mirror tiling? I mean really mirror tiling, not just flipping image vertically or horizontally, because you can already do that using negative scale. If you really use mirror tiling, not just flip, meaning your texture actually mirror tiles on the object, then it tiles in the first place, so you really don't need to care how many tiles you'll see, because it will depend on object's mapping anyway.
Does something like this, that one uses like once every two months, and can be fixed in less than a two seconds by setting two values to 0.5 instead of 1, really need another checkbox?