Three phase is three phase - it always requires 3 conductors. Granted one of these conductors can be grounded. I am not aware of any utility distribution system that uses the earth as the sole return path for a phase conductor.
Also the most accurate comparisons would be between the use of multiple single phase transformers versus a single three phase unit. Advantages may include (but are definitely case dependent):
smaller support structure
lower purchase costs
better utilization of inventory
reduced number of spare parts
