I think the key would be to make sure that people are paid not too much so that the low paid, menial but essential jobs are still filled and inflation doesn't sky rocket.
Guns, thugs, police, arrests. That kind[1].
>If people want to unionize they are free to do so.
Well they're not[2].
[1] -- http://en.wikipedia.org/wiki/Goon_squad [2] -- http://en.wikipedia.org/wiki/Union-busting
Oh, and then there's the issue that property in the means of production is itself a relation created by the capitalist state, as are the modes of its organization (ie: joint-stock companies instead of commons trusts or workers' cooperatives).
These jobs are low paying because competition is high between low-skilled candidates: "if you don't want to mob the floor, you're fired, I'll find someone else".
Do these people have a choice ? They usually have a choice between many low paying jobs.
Is it morally justifiable? If you believe meritocracy is a bad thing, then no. If you think it's a good thing, then low paying low-skilled jobs are just a fact.
That is another form of currency.
At the same time these kind of jobs are increasingly being replaced by robots.
You don't need to worry about that. If those jobs are really essential, then they won't remain low paid; the salaries will increase until they find people willing to do them.
Of course, it might be weird to see dishwashers earning more than programmers, but on the other hand that won't last, since it'll create plenty of business opportunities to automate them.
In my Grandfathers generation, attendants pumped gas into cars. People pump their own gas now. The world didn't end.
I've never seen a shoeshine boy other than in movies.
I expect a lot of jobs would simply disappear and no one would really mind. So you'll have to bag your own groceries, well, OK then. Hmm this store doesn't have any garbage cans or bathrooms for shoppers, interesting, I guess that means they don't have to maintain something that isn't there. My local bank branch no longer has tellers, its all on the insurance agent model where the local branch is a cheap one room office in an office park, or its all online, I was a customer of NetBank for many years until their downfall, which was nothing to do with customer service, they were bought by a subprime mortgage broker trying to diversify who obviously went bankrupt, no branches was never an issue.