Inflation is not the only reason why I want to spend/invest money now rather than save them. In fact, it's not even a good reason, because it forces me to spend money.

Deflation doesn't discourage spending by itself. The price of holding to the money is the lost opportunity of investing those money into something else. I'd say, generally saving money is a good thing rather than bad because it allows me to only invest them when I consider the opportunity good enough. Presently, inflation punishes me if I save money. So, if we talk about particular individuals saving money, inflation is not a good thing for them. It might be good for specific groups of people who receive the money I spend, but then one might say it is good for them at my expense. Which brings us to the idea that inflation is simply a tax.