What's "best" is subjective and up to the individual to determine. No one has the right to tell anyone what to spend the fruits of their labor on. You might think it's best for your neighbor to invest in some stock. He might think it's best to buy that new TV he really wants. Remember, the economy is not an ends in itself, but a means to provide us with material wealth.
As for their influence on the economy, if they end up giving a disproportionate amount of their income to payday lenders, great; now the money is in the hands of someone who does know how to invest it. Is there a problem here?
If someone puts their money under a mattress for 10 years, that just means the value of everyone's money increases, until that money is spent.
Also, spending for the sake of it doesn't grow the economy. Savings and capital does.