The issue is that it won't necessarily get spent, saving rate goes up with income which produces less currency circulation. The whole idea of trickle down economics just needs to go away.
If there's going to be government intervention in the economy (and there's going to be government intervention in the economy) then supply side economics works great in environments where there isn't enough investment.
That said, we don't live in one of those environments. We've probably had too much investment for the last 104 years or so...
It circulates but it doesn't circulate locally and it doesn't circulate quickly. The earnings of people with lower incomes tends to get reinvested directly into their local community. Higher income earners will pass their money off to a bank which may reinvest it into a local business but is as likely to invest it overseas.
Even if that's true, does it matter? Whether a dollar saved is spent locally, or saved in the bank and loaned locally, or saved in the bank and loaned to some other community, or wire transferred to family in Mexico ,or invested in a factory in Indonesia, some local community benefits.
That said, we don't live in one of those environments. We've probably had too much investment for the last 104 years or so...