If you have the tray icon enabled, pidgin should start up with the buddy list in the same state as when it last exited. If you don't want buddy list hiding behavior, you could always disable the tray icon.
- Bug#782689: pidgin: Buddy list not visible in GNOME 3 u
- Bug#782689: pidgin: Buddy list not visible in GNOME 3 Ari Pollak
- Bug#782689: pidgin: Buddy list not visible in GNOME 3 u