GeistGeld and SolidCoin offer block rewards that don't decrease over time, I believe. Bear in mind they also have other major differences to the design of Bitcoin though, especially SolidCoin (because it's not a community driven open source project).
The advantage of having a limited number of units is that it is a better store of value. On average, a unit of currency should only increase in value over time. Because we can break one bitcoin down to 8 decimal places (and further if necessary), there's no need to have it any other way.
When it comes to hoarding, the current exchange rate takes into account likely future increases in value (as well as potential risk of things like a collapse in value).
These questions examine the impact of hoarding/speculation on Bitcoin:
