I like the idea of custom games because you can truly explore a map and prepare your fireteam, especially for events like ToO. I would always do this when a new Halo was released. I would run through each map before I dove into multiplayer so that I would at least have a basic idea of things like weapon spawns or choke points. Edit: Additionally, custom games would be a great way to test guns before you bring them into the Crucible.