A currency that on average increases its value over time means creating an income flow without any risk. So all other investment options have to be that much more attractive to offset their attached risk.

There will always be some investments that are worth it and there will always be some reasons to save instead of spend, but the interesting things happen in between these two extremes. The inflation-to-deflation point is special because once there is deflation, it is not sufficient any more for a business venture to make money to be attractive.

If the currency under consideration has wide spread adoption, deflation means a system-wide slowdown in investment (_averaged_).