I've never quite understood it either.
I've volunteered a local scenic railroad some, and have been able to fire their small steam loco under supervision. One of the things the head fireman drilled into me was that a properly fired loco doesn't smoke. Shoveling coal into the firebox is sweaty, back breaking work even in a small locomotive, and every time I'd get smoke he would chastise me with "there's your hard work going up the smokestack." Thinking about the effort to build the fire and then waste it really puts things in perspective. We did intentionally smoke when we had a photo run-by since that's what people wanted, but that was it(we also liked cold days for the same reason since we'd get enough steam condensation out the stack for folks to get good photos.
I love AlCo diesels, but they can also be frustrating to run on a scenic line(where you're watching every penny) for the same reason. Our EMDs(two stroke) got a lot more service than our ALCos for a variety of reasons, partially because we have a relatively "new" GP-38 that's reliable(with lots of support since class 1s are still running some GP-38s and the similar SD-38), but also because the two-stroke EMDs are a lot more efficient in our operation. We're on the throttle a lot and usually between notch 2 and 4. The 4-cycle ALCos have massive turbos, and every time the throttle is notched up you run into serious turbo lag and they will belch clouds of smoke for a minute or two until things settle down. With long runs on a Class 1 where you're usually in notch 7 or 8 it's not a problem, but again we're on it a lot more(and at lower speeds) than on a main line. For that reason, our ALCos mostly sit idle these days.