It is not a tax. A tax is required and usually forced by a government. Why does anyone join any service or buy any product? Usually it is because they want or need something they don't have. There are millions of people who make an irregular income. I doubt many have a emergency fund, but many would benefit from having a regular minimum income level.
Maybe you can get an insurance policy to even out spikes of income or have the knowledge and drive to invest your money, but there are plenty of people who can't and don't invest their money.
Comingle is an interesting idea that would act as a pseudo emergency fund to provide a stable week to week income for their users. It could act to stabilize your income if you have an irregular income or as an backup plan for when you lose a job or income source.
Disclaimer: I am not affiliated with them. I just got this in an email newsletter and was intrigued.
Actually that was my bad. They use guaranteed on their website (which is a more accurate descriptor) and I got it mixed up. Either way it is less conditional than most jobs.