P/P's are of course cheaper to buy and cheaper to operate when one doesn't factor in the cost of the engine. Beyond that I'm not real sure if they have any other advantages over a MU. I guess the fact that you can take just one p/p car out for servicing could be considered an advantage too. MU’s must operate in a pair, so if one car needs service, its mate must go with it even if it doesn’t have a problem.
While it's still a long way off, if it ever happens, the overall regional transportation plans do include a proposal to link Penn with GC. This would allow for NJT trains to reach GC and Metro North trains to reach Penn. It would also decrease the number of passengers hitting the subway to get from one station to the other.
The real dreamers actually still hope to see NJT trains run up into Westchester on MN and out into Long Island over the LIRR. They also want to see the reverse of that, with MTA trains running into NJ. This idea of course while nice, is an extremely long way off. First of all you need the cooperation of two states, multiple agencies, and then you've got to deal with the incompatible power sources with the three transit companies. Currently not even the LIRR and MN can share MU consists, even though both are owned by the MTA. The reason for this is MN uses a third rail where the shoe runs under the rail. The LIRR, like the subway, uses a third rail where the shoe runs on top of the rail. Then of course NJT doesn't even know what a third rail is. Everything they have that's electrically powered is cat based.