Increasing the stake reward to 10 clams/minute would probably have an averse affect on available clam supply at the market, which is pretty healthy right now. I expect many clams would be pulled off Poloniex (and make other lower volume markets even thinner) because they're not staking there. Right now it seems like not staking is acceptable, but if stake supply went up 10 times, holders wouldn't want to get diluted quickly.