"At what point, if any, does Murphy's defensive troubles at 2B become a liability? "
When and if they ever do. Just maybe they don't. Even more likely they don't outweigh the value of his hitting. Are you even being serious?
"His inability to turn double plays almost cost them this game though. Big late inning double play opportunities come up like that quite often."
How many games has he cost us? Maybe if he ever costs us ONE we can begin to talk. Your act is getting stale.