You are not wrong in your thinking about getting interest on the savings. However, the interest rate is 4% per annum on the savings. That is really a tiny portion of our savings. It is also beneficial only if we are holding a large amount of satoshi or whole bitcoin in savings mode. Then only we get better interest on our savings.
Well, taking the won prize into consideration (83080 satoshis), and the fact @ 0.01369863% interest is earned over it in a daily basis, it means the prize won from WoF has guaranteed me a passive income of extra 11 satoshis every day. In the way I'm thinking right now, I guess it's better than waiting for 50 satoshis reward from WoF that might just not happen. I have already been greedy before waiting for too long and wasting a good opportunity of using my spins, and I prefer to not repeat the mistake... 40 satoshis base reward is a decent sum, anyway. I just think higher prizes are coming too rarely.