Small controlled inflation tends to be favorable both in real economies and virtual ones. Why? Because if deflation happens it creates a disincentive to use the currency.
If I know that my dollar which can buy a widget today could buy two widgets tomorrow, I'd be a fool to buy a widget today. Hoarding currency becomes a viable option for wealth generation instead of actually using the currency by say investing in business (in the real world) or buying materials to level crafting to increase income potential (in the virtual world).
If I have a desire to earn more Gil and I'm debating whether or not leveling a profession is a smart investment of my Gil, if the currency deflates, it may not make sense to bother with investing in leveling a profession since I'll experience real gains in wealth simply by hoarding my gil. Conversely, if there is a small but controlled level of inflation, it creates an incentive to spend on things like leveling professions, which improves the overall health of the economy.