Exactly, no one stays gone. They either run away only to get killed, leave but get convinced to return, or leave and come back because of a change of heart. Suppose it would be bad for the plot if they developed a character only to have them leave with no explanation as to what they're doing :P
Chloe Frazier on Uncharted 3: Drake's Deception. Her partner gets seriously injured during their latest mission and she quits it and tries to convince Drake to do the same since the stakes were getting way too high. While she does back down and is never heard from again, Drake keeps pressing on. You can apply the same to Sully in Uncharted 2: Among Thieves, but in fact he simply failed to meet the heroes in time to experience the rest of the adventure, since he said he would eventually catch up with them in Tibet claiming to be too old to be in the front lines with Drake against Lazarevich's army.
Regarding RE5, Chris tried to quit in Resident Evil 5, but was talked out of it by Sheva.