I think that this question, this discussion, presupposes that decoctions deliver a flavor impact. I think that while there is a large impact as a result of decoction mashing the portion of that impact that relates directly to flavor is small.
I think you get some carmelization and fuller flavor. Not a huge amount of flavor per se, but definitely noticeable.
When I do decoction mashes the beers they produce, to me, taste the same as stepped mash beers where I hit the same steps. I do think there is an impact of decoction. I like Eric Warner's reasoning in German Wheat Beer
A single or double decoction mash is used when brewing a Weissbier wort for three main reasons. First, it supplies the yeast with an adequate amount of amino acids. Second, it breaks down the higher-molecular-weight proteins [...] Third, [...] it reduces chill haze in the final product.
That's it, those are the only reasons mentioned. I would add two reasons - 1. Further breakdown of starches than with a traditional mash, ensuring full and complete conversion and, potentially, a higher extract yield and 2. The residual body benefits of a stepped mash procedure (really dry and yet really malty).
I actually heard Charlie Bamforth of the brewing with beersmith podcast say (paraphrasing) that the only reasons large breweries step-mash is to improve runoff and on a homebrew scale it makes no difference. Of course, he then went on to say that he has never homebrewed and tries to avoid drinking homebrew, so what does he know about it?