Like DmaxMaverick said, if alcohol was cheaper than diesel (by cost per BTU, not volume), there might be an economic advantage to its use.
Water/alcohol injection can be advantageous for other reasons (i.e. to cool the intake air charge). This can be helpful to reduce detonation in a gas engine, or it can be used to reduce EGT in a diesel. For either engine, its not so much about economy, but more about power.
That said, if someone was selling water/alcohol injection systems, and the diesel fuel costs at the pump were high, they might try to make the case about fuel economy to make the sale. Ask them to provide verifiable data... I'm just saying...