Here's how I see it:

(1) There needs to be an incentive for early adopters.   They are taking a risk because bitcoins might end up very valuable or they might end up worthless, depending on what the future demand for bitcoins is.  So there should some deflation built into the system.

(2) The deflation can't be too big, or it will discourage later adopters who will think they're being scammed by a pyramid scheme.   Also, according to some economists (hotly disputed by others) too much deflation can cause hoarding meaning fewer transactions being made in the currency.   Is it true?  Hopefully we'll get to find out.

Corollary to (2): if you don't like the deflationary monetary policy of the current bitcoin currency, or some other feature of it,  feel free to start your own and see if you can win over the merchants.