Start with the basics: what do we all, more or less, agree on? A job should allow a person to live. If someone works 40 hours per week, or whatever a 'full time job' requires, they should be able to afford to feed themselves, clothe themselves, and have a place to live. I think we even agree that at some point that person should be able to retire.
Is that a basic tenet of modern life? If so, where does part time work fit in? If someone works two jobs at twenty hours a week, do they deserve the same thing as someone who works 'full time'? I would assert yes. Which means two things: one, we can't treat part time work differently than full time work - any amount of work at any number of jobs should, if added up to 40, give the same benefit. But - and here's a catch - if we do that, we need to move a lot of things off the employer. Things we might not expect - like sick time and holiday pay (if we think those are things people should have). Health care (no question there - especially as it gets more expensive, we can't expect employers to foot the bill). Obviously retirement. We can let employers add to the basics to attract workers, but whatever we consider the minimum should not be left to employers if there is any way for them to opt out. Otherwise, as a rational business, they must opt out.