The decoy genome is an improved human genome from the regular reference. It's faster for mapping and it reduces false positive rate.
In this case, there shouldn't be any reason to use the regular human reference, because the decoy genome is simply better and faster. For instance, it has DNA sequence for common virus genomes presented in humans. It has everything in the regular reference.
Q: What's the point of the regular human reference (e.g. hg38) if the decoy version is more accurate and is a better representation for the human variation.