There was talk of a second cache early in the mission design (sometime before they exited the crater), but from what I have seen in recent reports it seems like the post Three-Forks samples will be retained on the rover for the time being. Main Pro is the rover can deliver all the tubes to the SR Lander without needing the fetch helicopter to return them from another sample drop. Main Con (IMHO) is the sample return mission was originally 2 sample helicopters, because of budget that may be reduced to 1, to me that is a huge and unacceptable risk, one fault takes out 1 helicopter and the entire SR mission. The Mars rover's hardware for driving and its robotic arm have performed very well on MSL (>11 years) so I believe they will keep them on-board for the time being, but review the situation when it is time to launch MSR, and that is many years away...