50 cent and $1 coins are not commonly used in the US due to decisions of the companies that made coin acceptors (the part of the vending machine that takes, counts and validates coins) back in the 1950s and 1960s. Basically, they did not want to hand out 45 cents of change from a 50 cent piece (nor 95 cents change from a dollar coin) so they deliberately chose not to support those coins. Consequently, they stopped being used by many in the public as they were "inconvenient." As those coins became less common, the companies that made cash register drawers chose not to make bins for coins that weren't in common use, so there became no easy way for businesses to take those coins.
The low usage of $2 notes is also reflected in cash register drawer designs - there is no place for those notes, so most cashiers toss them under the drawer along with checks and large ($50 & $100 notes), so the circulation of $2 is very low. I personally like to get stacks of them and spend them instead of carrying $20 notes around. Some places, like at the Renaissance Fair, hand them back as change to the next guy (there is no slot in the till for them) so the next guy gets something they've rarely seen and they get thrilled.
If you mean day laborers or migrant workers, I've seen first-hand some employers now pay them with rechargeable AmEx or Visa cash/gift cards like Green Dot. Keeps the shift supervisor from having to carry conspicuous wads of cash around, and it's directly deposited on their card at the end of the shift, so no need for lineups.
Hm... I'm thinking of something universally accepted, something that doesn't need relatively expensive handling equipment, something anyone can get and anyone can exchange for goods and services...
It would be interesting to see the effects on these "industries" if we were to move to a purely electronic exchange of money.
Edit: Here you go: they are polypropylene polymer. Wild
The trick is no one actually wants a dollar coin.
Apparently there's a difference in priorities. Some people prefer an illusion of no inflation with a greenback as strong as ever. Others might prefer making the high velocity currency units durable and easy to handle.