The problem with using the approach frequency for B and C airspace is that there may be several frequencies to choose from. For example if you're approaching ORD from the South you're in the airspace for one TRACON controller, if you approach from the East you're in another, etc. These frequencies will change with differing times of day, traffic load, and weather/runway configuration. These changes are also not likely to be found on a chart, and if they are, you're not going to know when that facility has combined, or not that frequency. The approach frequency for MDW, on the other hand, is rarely used. The ATIS nearly always says "...contact MDW approach on (tower frequency)..." Again, there's nothing written down on charts, or in the tower that specifies when they will be using the approach frequency. It's the judgement of the tower supervisor, based on current and predicted traffic load, staffing, weather, etc.
I can't imagine that these two airports are the only ones that operate in this manner.