You beat me to it, DT!
Same here.
When Monogram released its B-17G, I got it for Christmas. My best friend, who lived next to us, got it, too.
We both started building the kit the day after Christmas. I looked forward to a fun project over Christmas break.
A day later, my friend said he'd finished building the kit and showed it to me. I asked if he finished the interior and he said, "No, no one's going to see it. Why are you finishing that stuff?"
And that was my answer, too: "I know it's there!"
My first experience with the question.
Seriously, back then, with the kits as they were, and the instructions written the way they were, naming parts and things like that, I learned a lot about the subject-what the crew spaces in a B-17 were like, or the fighting compartment of a tank, the bits and pieces of a V-8, from completing everything in the kit.
Today, I may decide to omit something in the interest of time, weighing it against other factors in the project. But very seldom, since I build for myself, rather than for a customer.