At the end of Tron Legacy, Sam Flynn and Quorra escape with his father's ID disk. Why are they so reluctant to leave Kevin Flynn behind when it is clear that Sam could simply get out, leave Quorra there, and come back in to rescue his father? The portal only closes from the inside, not the outside.
You can only exit with Flynn's disc. Sam's disc doesn't contain the information necessary to exit the Grid. We know that because when CLU examines Sam's disc he says he was expecting more and gives Sam back his disc. If Sam's disc had the ability to let him exit CLU would have kept it.
Also Sam didn't have a disc when he entered the Grid. It was only given to him after he was captured as a rogue program.
The portal is open only from the outside, and for only a limited amount of time. Once it closes no one inside the grid can open it again. Only someone from the outside can open it. That is why Flynn was trapped inside the grid for so many years.
Sam doesn't know about the portal until he finds his father. At that time they realize there is very limit time left to escape the grid. They all must travel across the bad lands to get to the portal before it closes.
A disk is not required to exit the grid. Any human can exit via the portal and ISO's are more like humans then programs so they could also exit, but I assume they didn't know that until Quorra actually tried to leave.
CLU is a program and can't exit via the portal, but there was something on Kevin Flynn's disk that solved that problem. Not only for CLU but for all programs to exit.
The key to stopping CLU was to get Kevin Flynn out of the grid. As long as Kevin was in the grid CLU would be hunting for him. Sam could have left the grid to come back later, but that wouldn't have solved anything. CLU would still have been hunting down his father's disk. Getting his father out of the grid was the only solution to stopping CLU.
The only other option was for Kevin and CLU to merge again. That would have stopped/destroyed CLU, but also kill Kevin in the process.
Had Sam left the grid he would be abandoning his father to his fate at the hands of CLU. Sam had already exposed where his father was hiding. His father had no where else to go, but for them all to try and to escape.