Your question about using both bar-codes and RFID book ids should generate many responses. There are several ways of looking this question. The points mentioned below are only my opinions. While they are accurate, there is probably more than 1 way to view the information.
1. If you are a consortium that has some libraries with RFID and some without and if you are intending to inter-loan the books between these libraries, both bar-code and RFID are probably a necessity.
2. If you are a consortium that is converting all your libraries to RFID, it may be that it will take a year or two to complete the process (budget is my point here, not tag conversion time). During that conversion period it would be useful to have both RFID and bar-code functionality.
3. If the bar-code and the RFID book id are identical, then you have a ready-made "backup" of your RFID tag should it become corrupted.
4. Erasing RFID tags with NFC-enabled phones is still a possibility with the ISO-15692 tags. This is not to cause alarm, it is just another reason to consider a bar-code so you can easily rewrite the RFID tag.
5. Some libraries are considering a hybrid security system. They utilize RFID tags for those books that will be loaned, activating or deactivating the AFI /EAS alarm. If the library was already equipped with EM (Electro-Magnetic) gates, the library could utilize both EM and RFID. The library will keep the EM "tattle-tape" inserted in the reference and antique books, which are not to leave the library. A third opportunity for hybrid security would be to protect high-theft items (i.e. music cds and movie dvds) with the tattle-tape while utilizing RFID for the issuing procedure. Note that you may install an RFID tag inside a reference or antique book for inventory purposes, while using EM for security.
6. There are RFID tag printers. They will write the RFID ID book id to the tag and print the bar-code on the front of the tag. So if you decide to go down the duo-path, you will be able to easily create both identifiers in one pass.
7. For the purpose of library identification id, I believe you could use an ISIL code to identify your library within RFID data-model. The ISIL code could also be a part of the bar-code.
8. If you would like to think of the mobile phone as a resource, only a handful of models have NFC readers in them but all smartphones can read bar-codes. If a user would like to get detailed information about the book, he or she could perhaps scan the bar-code or RFID tag with a mobile phone, linking him to a database about title, author, video presentation, etc.
9. What if you move from 1 LMS vendor to another in the future? Will the tag as it is written be readable by the new LMS? With good standards in place the answer should be yes. However, there is the chance that the current LMS has written it in a unique way. A bar-code would assist with the secondary conversion process. By the way, I am not suggesting that you scan the bar-code and write the tag for your entire stock. During the bulk conversion phase, should it be necessary, an application could be written to assist. The bar-code to RFID tag conversion is a onesie-twosie conversion that would handle all books that were absent during the bulk conversion process.
I was trying to think of a 10th point to round off my list but nothing comes to mind. :-)
Best of luck with the RFID installation.