I find it helpful. If it's a program that generally supports single dash options only, having it respond to --help will falsely make me believe that it's one that expects double dash options.

Letting me know that the proper way to ask for help is `-help` also communicates that GNU style double dash long opts is not the way to go with this particular program.

Then I suppose you could argue that GNU style double dash options should be supported. Meh, I dunno. They were the ones that invented the fifteenth standard (xkcd/927).