The use of flash for defensive and offensive reasons for me depends on the time of the game and my team composition and enemy composition. For me I would only use flash offensively early game if I am 90% sure I can get first blood in lane or a quick gank out of jungle to finish a kill(mainly with Shaco,Lee Sin, or Skarner for me) that grants me a kill for some extra gold(Refrencing before jungle remake). Late game I mainly save my flash for defensive purposes just in case I do something stupid or end up with bad team positioning entering a team fight. I say this because if you think about it if you are 40+ mins. in and the enemy team kills you then it will be a 4v5, resulting them taking Baron and have map control.(Depends on warding,lane push,and towers.) Then they would push towers down and inhibs resulting in a hole for your team to climb out of that might not be easy to do.