I think it can be within the bounds of realism to have multiple 0.5 frequencies to an airport.
Example: Madison and Milwaukee are very close together. I have a bunch of planes doing flights all day out of Madison (whole number frequencies). At the end some still have an extra hour, enough for the quick hop to Milwaukee, and not enough for any other profitable route that I don't already fly. So I give them all 0.5 frequency flights to Milwaukee to make a bit of extra money. In real life, that would just mean I have several planes overnighting in Milwaukee each night.
The real departure from realism is when you have multiple 0.5 frequencies per plane. That would mean it's overnighting in multiple places, which is obviously beyond the bounds of realism.