With no inflation, my risk free investment of money under a mattress produces no return. My risky investment with the company has the probability of a positive or a negative return.
With inflation, I'm forced to make a risky investments to break even. True, I can no longer keep my money under a mattress but I'm still an unsophisticated investor and choose to park my labour in houses for example which inevitably leads to a housing bubble.
With deflation, I agree, people would rather not invest in a business producing a lower return than the rate of deflation but why would that deflation occur? One reason could be more goods chasing the same amount of money implying real economic growth.
The problem I have with the current inflationary system is the distribution of new money, if all currency held and prices denominated in that currency increased by the same factor. It would result in a nominal increase in prices. No one would be richer or poorer after the increase. The current system distributes new money to people who directly interact with the central bank, rewarding them at the expense of everyone else.