Very simple, if you buy shares with BTC and BTC price goes up significantly before your shares mature, you might be worse off regardless of interest you get from Bitcoin trader. On the other hand, if the price goes down after you purchase shares, you might make more money.
Well, there is another option if you believe that is the case, which I did on my first 2 runs.
I deposited 1 bitcoin, and then I used the currency converter, converted the bitcoin to OKPay cash balance. I then moved my money from OKpay cash balance to OKpay e-wallet and bought shares.
And after my first shares expired I had the EXACT same balance as before I bought shares in my OKpay e-wallet, which I then moved to OKpay cash balance.
How ever, there is a fee of 2% when converting bitcoin balance to OKPay.
That said, Im on my 3rd run now and I have bought my shares with bitcoins for a value of $2000, but when they expire I am sure that I will have $2000 in my e-wallet and when I withdraw them to my bitcoin adress, they calculate from bitstamp ticker how much I'm getting in btc.