Their justification attributes a blog post where the author basically says "don't use tools that will break your code if you omit these syntax features". It encourages people to write code that will break when used by others, dictating the tools they can and can't use. It's the equivalent of a child closing their eyes and sticking their fingers in their ears to magically whisk away the person standing next to them.
It's pretty disappointing to see GitHub, a company that a lot of developers (including myself) look up to and will follow, espousing something like this.
the only tools that will break your code if you don't use semicolons are so old they shouldn't be used for other reasons. At this point, you should really be using either yui compressor, closure compiler, or uglifyjs for reasons that have nothing to do automatic semi-colon insertion or not.
If you are still using something like packer, that bit of advice will take about 5 minutes of work, and give you significant decreasing of file size.
Semi-colons need to be in 1 place, the beginning of a line that starts with a (, since that is the one place that automatic semicolon insertion will really screw you. If it makes you feel good to write a semi colon at the end of every line I don't think anyone is stopping you, but it isn't the language or tools that require it. You can also feel free to explicitly wrap every statement in parens (because it is necessary in a few cases), or end each line with a // after your semi-colon, all these are freedoms the language gives you, but are as necessary as semi-colons.
I haven't been using unnecessary semi-colons in js for years, and have yet to run into a problem.
They actually don't provide a justification. That blog makes the argument that omitting them is harmless. Neither GitHub nor the blog make an argument for why you should omit them, however.
Do you use semicolons in Ruby, Python, shell, or other semicolon-less language where semicolons are allowed? If not, what's your justification for omitting them?
It's pretty disappointing to see GitHub, a company that a lot of developers (including myself) look up to and will follow, espousing something like this.