The next argument to come up might be, "Well, if I was the only one to have a money tree, then I'd be richer." This would be true in the sense that your purchasing power in proportion to others would increase because money is used to represent that purchasing power (and you would have more money). But, the problem is that you didn't actually produce anything extra. You would merely be stealing the production of others and spending it as if it were your own. This would be no good for society as a whole (not only are you not producing anything, but you are taking production away from others... this results in a net loss. Wealth is consumed without adding any new production). If (before the money tree) your money represented 5% of the economy's wealth (and you had 5% of the money supply) everything would be fine. But if (after the money tree) you had 5% of the economy's wealth, but had 10% of the money supply, then you would be able to spend as if you had 10% of the economy's production. You would be stealing savings and production from those who have legitimately earned it.
This is what the Federal Reserve does. They are that money tree. The Fed prints money out of thin air. It might not be a problem if the newly printed money were distributed to each person according to their current wealth and their future production. In this case everyone's purchasing power based on their legitimate contribution to society would be preserved. However, the money that the Federal Reserve prints goes to the central banks and is then given to banks across the nation to loan out to people and businesses.
The distribution of the newly created money goes out to the first people that get loans (most often this is businesses). The spenders are rewarded and the savers are punished. The spenders are spending money that isn't actually theirs to spend. As a side note, see this wonderful comic book for the reason why prosperity is built from savings and NOT spending.
So why does the Fed do this? The idea (at least in a stumbling economy) is that an increased money supply given to banks would drive down interest rates (which would boost the economy- i.e. give it a stimulus). If all banks have extra money to give out, then anyone could get a loan from a bank. If the first bank has too high of an interest rate, they could go over to the next bank and get a loan with them. In other words, the surplus of money to loan out (that was printed out of nowhere) would lead to bank competition and lower interest rates for borrowers because there would be so much to lend out.
With the lower interest rates, more people would be willing to invest in things (demand is increased based on this fake money and fake level of interest rates). More specifically, more people would be willing to invest in things they wouldn't normally invest in (if they were to base their decision on their real wealth, their real purchasing power, and the real level of interest rates). This is what is called malinvestment. People and businesses are tricked into investing (and spending) their money on things they can't actually afford based on their real wealth level (they focus on their amount of money from the money tree, not their actual underlying wealth). The excess spending leads to a bubble- a short-term boost to the economy based on the excess spending. Real savings are depleted and once people realize they are spending money they don't actually have because the wealth underlying that money doesn't actually exist, the bubble pops.
The solution to this problem isn't more spending. It isn't more stimulus. The solution is to get rid of the money tree. Let everyone spend money and invest based on their actual wealth and their actual production. "Stimulating" the economy by creating artificial wealth only leads to disaster... see the Great Depression... see the Internet bubble... see the Housing bubble.
