Posting the definition for expedient the noun was superfluous. I used it at as adjective. I think it's an accurate word to describe to the graduated income tax. 
It's easier to get people that have more to pay more, but the value of services provided is in no way proportional to income.
To clarify, I'm not defending the flat tax. I'm proposing a flat fee: budget divided by the number of payers. It would be cruel, but it would be fair. My point is, people aren't interested in fair.