The reason why it's a sin:
The person who transplants an engine that's lesser than what they originally had is just plain cheap.
They make a perfectly desirable car all of a sudden undesirable. There is no sense in that.
That's not to mention that they f@#$ed it all up in the process, and now see where the cheapness has gotten them; they're going to end up paying more just to fix this mess than as if they just fixed it right the first time. This is where the problem lies; even if the engine is a similar engine, such as an M54B30 but without cams, the ECU is programmed to work with an engine that has cams. I'm sure the ECU isn't the only thing that was designed to work a certain way. Even swapping the ECU won't work properly; there is probably a tamper dot present on the cluster, effectively making the car's value drop like a rock.
Bottom line is, you can't just swap in a lesser engine and expect it to work. This is where the cheapness comes into play.
Fix once.