>I think that busses are only included in the system due to the convenience of not having to have a line to the depo

Busses are just a lot more flexible in general. The costs with starting and stopping bus lines are much less than other forms of transit, the vehicles tend to be more interchangeable, the infrastructure is less substantial, and so on. I'm not saying I generally enjoy riding busses, but the costs to the system are very different than other forms of transit.