I do tend to explain DEs when someone asks for a distro recommendation, because yeah, I also think that it does make the biggest difference for desktop usage.
However:
- You can't download an ISO for a DE. Noobs want to know what to download.
- Given that you can switch DEs relatively easily, the choice of distro can carry more weight.
- Depending on what kind of user they are, I do think the distro makes a difference. For example, generally I wouldn't recommend Debian-based distros to a programmer. Or plain Arch to someone uninterested in technology.