It's cheaper to install if you're paying for the primary since you only need to bring in two phases. If you're a mile back in the boonies, that can be considerable.
The biggest disadvantage is that its voltage regulation under load is poor compared to a 3-pot bank. This can lead to serious voltage and current imbalances in 3 phases devices, which most of the common heater type protection won't protect against.
I had a water well rep tell me once that he had heard that some pump manufacturers would void their warranties if they found out the pump had been on an open delta.
