I'm sure they're very rewarding for the author/editor - but the cost is externalized onto unversed readers, this is generally considered bad practice in other areas of programming.
The good thing about ligatures is that they only affect how the text is displayed, so people who don't like and use ligature fonts won't encounter them.
The exceptions may be, as you said, online tutorials etc. with their own fonts and formatting, but I think that's more a part of the problem of bloated website design overriding browser-wide preferences.
For me ligatures are less important than syntax highlighting, but I do find them helpful - especially the greater-equal and smaller-equal signs are much more readable with ligatures, and that also applies when I read other people's code in my editor.
Not really, too often on the web it is the font decided by the author or an intermediary - the above page is case in point or they wouldn't have been able to demonstrate it as I have never explicitly opted-in to ligatures at any point.