I visited 4 cathedrals from 6 in total: (1) S Nicola di Bari (extremely impresive exterior&interior with the tomb of St. Nicholas), (2) Cattedrale di Bari (massive buildings with large number of fine romanesque details), (3) Cattedrale di Bitonto (I would say the typical example of romanesque Apulian Church, though the refined version of S Nicholas, with rich decoration and excavated fine floor of its predecessor with famous mosaic of Bitonto Grifone), (4) San Corrado in Molfetta (old cathedral - there is alse a new one in the town; it is quite small, maybe not such impresive but it has byzantine structure with three domes in the main nave). What I know from the literature the quality of Catttedrale di Ottranto can be found in the unique mosaic floor, and Cattedrale di Troia has decoration inspired by Tuscany style such as Pisa and it is famous for the rosetta window in the west facade.

IMHO I would include also Cattedrale di Trani to the list - I strongly recommend to visit Trani and spend there a couple of days if you are in the area, it is really amazing place. In this case, the cathedral has been altered in gothic period so it is not pure Romanesque. However, most of the proposed churches such as Bitonto (and even S Nicola) is the result of extensive restoration to mediaval appearance...

In conclusion, I think that this ensamble deserves to be included to the list, because of its quality and merging together styles from other parts of Italy (Pisa), Europe (Normandy, France), and Byzantine culture.