Originally Posted by Scifience

I've frequently had this request; it's usually because the agent needs it for APIS info, or to make sure ticket details are correct. Sometimes, it's also to arrange things like visas on arrival (if they are coordinating) or other value-added services.

Assuming the agent is reputable, I wouldn't worry too much. Given that many hotels, banks, SIM card sellers, etc. all keep copies of passport details as well, your data is likely floating around many places already—why lose sleep over one more?