Is a bleeder-valve to let air into the water line necessary when the plumber put in a frost-free valve? Should a bleeder-valve leak if kept open?
IMHO:
In this case, a bleeder valve is used to protect outdoor faucets from damage during freezing weather.
Pre-winter, you turn off water to the outdoor faucet, you open the outdoor faucet and then open the bleeder valve to let the water drain out.
In the summer, you close the bleeder valve, you close the outdoor faucet and then turn on water to the outdoor faucet, restoring it to normal use.
I'm guessing you had the outdoor faucet replaced with one that keeps the valve portion inside the house (where it's warm) while the handle of the faucet is still outside.
They left the bleeder valve in place...but it was in the open position...oops.