Earlier quoted context omitted.
No, there are literally no good reasons for that. If you work a full-time job, and I don't care what that job is, your employer absolutely should adequately compensate you, for the amount of time spent on your job. Your employer is buying your time, the most precious and absolutely non-renewable resource you have. Don't sell it too cheaply.
Is being paid a non-living wage better than being paid nothing at all? Say I am a one man shop. I would like an assistant, but I don't actually make enough to pay the assistant a "living wage". My options are to hire an assistant at non-living wages, or not hire an assistant at all. Which is preferable?
Have your assistant come in only 2-3 days a week or or only 3-4 hours in the morning.