Databases

Databases (like MySQL and Postgre) are not as fast as binary when they are both done right, but they are much more flexible and very easy to use. First, lets start with the ease of use. You can easily add, remove and change data in a database without having to worry about the rest of the data being affected. You also get a very nice, easy to work with structured and sortable view from the database GUIs (like Navicat and SQLyog). This saves you a lot of time from having to make editors to edit your information.

Databases also make editing / viewing files remotely very easy, so you can display information from the database on your game's web page, run multiple servers that access the same database, and edit the game's information from any computer.

When you branch into the world of multiple servers accessing the same data, using binary files is going to require you writing your own server that will give the requested information, which will take a lot of extra work and greatly increase your development time.

If you have never used a database, I highly recommend it. I was very hesitant to use a database to store information for my game until I actually played with it a bit, and am very glad I did. Information that doesn't change while the server is running (NPCs, objects, quests, etc) are all loaded from the database at server runtime and saved in binary for fast access, while information that will change (in-game mail, user accounts, etc) just stays in the database.