The risk with keeping your coins on SR is unexpected downtime. It's happened a few times already that the market went down for several days. Sure, it may not happen in the next week or the next few months, but the one time it does happen, you will experience head aches. This is not a problem unique to SR, of course. Ask the people who were storing their coins on Instawallet, or on blockchain.info the times it went offline. I'm only comfortable with storing my coins in my own client, with multiple backups.