The main place you will see 3p "wild leg" delta is where you have a a mix of light industrial bays that generally get along fine on regular single phase. When you have a customer who needs a little bit of 3 phase for motors you can give it to them for the price of one more transformer. You have the same basic single phase service you have in a home with one extra transformer and that may be quite a bit smaller than the single phase transformer for the 3d phase.

Greg Fretwell