Q: My librarian has asked for authorship, is that really necessary?
A: According to the International Committee of Medical Journal Editors,
If your librarian created the search strategies used for your systematice review and consulted with you regarding the best resources to search, that is considered a substanial contribution to the conception and design of your study. If your librarian took all of your search results and collected them into a citation management system like EndNote, deduplicated those results (removed duplicate citations), and possibly exported them into an excel workbook for you, that is considered a substantial contribution to the data of your study.
PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) requires that you include a thorough description of databases, other resources, and dates searched as well as at least one full search strategy. You definitely want to have whomever designed those searches draft that methodology. If you are planning to include at least one full search strategy in an appendix, it is only right to grant the creator authorship.
It is common for search methodologies used in systematic review research to be reported innacurrately. Don't let this happen to you. Make sure the librarian who did your searches reviews the final manuscript for accuracy.
Q: Why do I have to use more then one database? Isn't it all in PubMed/Medline?
A: No, it is not all in PubMed/Medline.
Gavel Y, Iselid L. Web of Science and Scopus: a journal title overlap study. Online information review. 2008;32(1):8-21.