Should you buy back the coins you spend on merchants? Is Bitcoin inflationary?

Someone on reddit asks me why I don't buy back the coins I spend on merchants:

If I treat bitcoin as cash, why do I need to buy it back? IE: if I spend money in Yen, why do I have to convert more in Yen? Holding bitcoin earns me no interest so I have no incentive.

Also bitcoin in inflationary, it is not deflationary. I have no idea why people say the opposite. Bitcoin just happens to inflat at a predictable rate/tapers off until its end of life. Deflationary literally means a shrinking money supply or negative inflation rate, which is obviously not true.