As stated above, once dispatched the MEL is for reference only. I will always refer to it though with ANY airborne fault. Most importantly it will allow you to assess the serviceability of the aircraft once you have landed. If the fault has a maintenance procedure associated with it you may not want to land at your destination if you don't have engineering coverage.
As for ETOP's we have an ETOP's checklist that we refer to for inflight failures, this takes precedence over the MEL. There are failures that the ETOPS checklist allows for continuation whereas the MEL may stop dispatch on an ER flight.