Ideally, I wanted a way to change out the engine. I figured with my space restrictions that a train engine yard would give me a lot of fun being able to swap out engines. Plus with engines there are ways to decouple the engine from the train when it is on the track without me having to use my hands.
Switching out cars would be okay, but decoupling becomes an issue, more space is needed, and I do not think I would find that to be all that fun.
Being able to swamp out whole trains was a thought, in G scale the length of the siding would have to be very long in order to run a long train. I could use shorter trains, but that is not as fun in my opinion. Plus my layout, is 120 feet of track, and I like to see longer trains.