If we kept inflation close to zero (or negative) wealth would tend to concentrate in fewer and fewer hands, and the bias would be toward idle rentiers. People like property landlords would get unbelievably rich by collecting mandatory continuous rent payments. Since deflating currency holds or increases in value they'd have zero incentive to do anything with it other than hoard it. Eventually you end up with a landed nobility.

Add to this the fact that deflating currency makes loans hard to service and biases the lending market toward default. This further decreases the incentive for those with a lot of currency to lend it since the probability of losing it to default becomes higher.

The ideal is probably a little bit of inflation. I think it's possible that we have too much, and that we are misinformed about that because we are measuring the wrong things. I'm not positive but I seem to remember that the CPI does not include things like rent and college tuition, which if true is utterly insane and makes it a bogus statistic.