"Ba dhochtúir é" implies that being a doctor was a permanent state of his. "Bhí sé ina dhochtúir" is more temporary; it could mean "He was a doctor (at the time, but wasn't one later)", and it could also mean "He became a doctor".
That's the form I also recall. The Caighdeán seems to support. It gives this example ceannaire maith ab ea í siúd but not clear when you would use this form as opposed to ba ceannaire maith í. Personal style?