The Secret Bases in Generation IV are found underground, and instead of NPCs being used to represent other players, the other players can be encountered face-to-face using DS wireless communication.
Players cannot battle each other for money underground, however, if a player enters another's secret base while they are not decorating (as that requires the entrance be closed), they can attempt to steal their flag and take it to their own secret base for rewards, from decorations to a Trainer card star.
Source (Except for my paragraph V)
So, cool thing about the bases are that you see real characters via wireless. If you catch their flags, then you can get a Trainer Star on your card, and people love to collect those. You can decorate and have fun in there, and dig up treasures like traps, to trick your friends and spheres are used to buy them! You can get shards and go to the move tutor with your shards. Plus its a lot of fun to do all of this!