> But, one's ability to buy things seems to be improved...

It's only improved if there is a lot of investment and new / more products coming out. deflationary currency promotes holding on to your money so that it goes up in value, but if everyone does that there will be less products / services and the value of the currency will ultimately go down. If currency has some inflation, then it's used as a transaction currency so that you want goods rather than currency, so the product becomes the important thing. If you have too much inflation however you get way too much money for the amount of products, so you want stable inflation.