In SBS Volume 82 it was stated that one of Blackbeard’s hobbies is historical research.
And in SBS Volume 88 Oda said that in the real world Blackbeard would be an archaeologist. As we know Nico Robin”s role in the crew is the archaeologist and she can read the Poneglyphs.
Nico Robin’s goal is to unravel the mysteries of the world, to discover what happened in the Void Century. It’s quite likely that Blackbeard would like to know about the Void Century due to his hobby being historical research. Plus it’s been said that the Poneglyphs contain info on the 3 Ancient Weapons which Blackbeard would be interested in. Plus the combined locations detailed by all four Road Poneglyphs allows one to locate Raftel.
So Blackbeard wouldn’t need to kidnap Robin to fulfill his purpose.
*Theory by shadowfire22