just to make my point clear
lets say only 20% of USD deposits (i mean ppl who put money in bitcoinica i dont talk about reserves) are used then you bitcoinica have to much money, the interest rate should be next to zero you want this ppl to use the moneys, the cost for maintaining a position should be next to 0 and if you can should be positive
now lets say 80% USD deposits are used then you should give a incentive so ppl make more deposits by increasing interest rates and also increase the cost for maintaining open a position
Well, it's different. The position swap is the difference between the buying rate of one currency and the selling rate of the other.
When we have enough USD, the buying-selling spread is larger, because more depositors share the fixed amount of interests (from position holders). Same thing applies to BTC.
Now, when you start a short position, you are selling BTC and buying USD. So you are paying the BTC selling interest and collecting the USD buying interest. In terms of BTC, what you pay will be used to pay the people who provide the BTC for you to sell. In terms of USD, you are collecting payments from people who use the USD you bought to trade.
However, the interest you collect can be less than what you pay, hence the negative swap.
We have the absolute incentive the stimulate trading activity so that we can earn more spreads (which earns us over 100x more than interest), but we have carefully considered the interest system to be the best solution to the reserve problems that we had in the past. In Economics terms, the demand for funds (position holding) is relatively price-inelastic and the supply of funds (deposits) is relatively price-elastic. Our aim is to ensure sufficient reserves while not affecting trading too much, and the base interest that we set exactly achieves this aim.