For me modelling is a time vs. a reward issue. I don't have a lot of time to devote to building models, and at some point I have to complete them or too much builds up on the workbench. Or worse, I start to lose parts.
I will put an engine in a model if it comes with one. I will paint it the proper color, but I do not wire or plumb it. I will do some basic detail to a chassis, but once again will not get carried away. I display my models on a shelf with the hoods down, so detail in the engine bay or the chassis will not be seen.
One of my most recent builds turned out to be a curbside. It was a Jo Han AMX. The engine and transmission were warped and there was a serious gap where the sides glued together. The amount of time it was going to take to fix was not worth it, so it became a curbside. A picture is below.