Acting as a short-term store of value is very important for a currency, but it's bad for investment if it's good as a long-term store of value. Money at its core is not a store of value; that's one of the requirements, but at its core it is a common medium of exchange. It is the very indirection that makes markets using money more efficient than ones based on barter; having a single common medium of exchange means you only need to solve the price discovery problem for n products rather than n^2.
Deflation is bad because it turns money into an appreciating investment asset; this means it reduces productive investments (i.e. those that actually incentivize people to create wealth). A (quasi-)fixed base of money as a root cause of deflation lets people with a lot of money to begin with siphon off wealth from people doing productive work that increases the size of the economy and the velocity of money.
TL/DR: deflation means theft of wealth by people owning currency at the cost of the economy at large that uses the same currency.
Deflation is bad because it turns money into an appreciating investment asset; this means it reduces productive investments (i.e. those that actually incentivize people to create wealth). A (quasi-)fixed base of money as a root cause of deflation lets people with a lot of money to begin with siphon off wealth from people doing productive work that increases the size of the economy and the velocity of money.
TL/DR: deflation means theft of wealth by people owning currency at the cost of the economy at large that uses the same currency.