They don't spend it mindlessly, they still allocate it relative to other opportunities. Low but positive inflation makes a lot of investments relatively more attractive, therefore incentivizing people to invest in economic activities, instead of sitting on their cash. Deflation creates a dearth of resource available to invest.
Plus it's a redistribution mechanism. If all you have to do is to sit on your cash for it to grow, then it's a rich becomes richer system. If you have to invest that money, there's an uncertainty, a risk, and opportunity for people to multiply their starting capital.
Some early bitcoins buyers are saying exactly that: "I got in early, I'm set up for life, if you didn't get early enough too bad you're gonna stay poor". It's not really how we'd like an economy to function.
> [...] people would have to spend it only on whatever they actually really need or desire. Is that bad?
It's bad if you want a growing economy. And usually technical innovation comes from relatively niche investments that no one really needs right now.