I agree, people will become rich very quickly with this interest rate. I'd be happy with the interest rate being 1% or as Chris said, even lower.
Sabin, the purpose of the store is to create a forum currency. You can use it to buy name effects and items for role play if I'm not mistaken. Eventually members begin to trade the currency amongst each other for sigs, avatars and other things. It's a good feature of the board because it encourages people to post more.
Gerard, I don't think you really need to return to the forum that often to collect interest. If you buy auto-collect for $50 it does it for you.