There are several reasons to perform an engine flush, especially on an engine of unknown maintenance. For starters I live in a medium sized city. I could not find the proper oil for my 2012 jetta anywhere other than the local VW dealer. I just ordered Liqui moly from amazon. But I'm a Technician at a power sports dealer, i did the research on why the 507 spec is used. The average person trying to save a buck wont care, just stick some 10w30 in there and go. I've also seen what older 1.8t look like even with basic oil changes under the valve cover.
I personally use atf to gas flush engines. I add up to one quart to the crankcase and run them for a while depending how bad it is. Then drain, and fill like normal. If its really bad I will rinse and repeat with cheap oil and oil filters until it starts to look good. Atf has lots of detergents in it and will help break down the sludge.
now our TDI's should never need flushed if they have a DPF and are regularly put into regen. The extra diesel fuel will act as a solvent in the oil and keep things clean.