The PJM Interconnection is what they call a "tight" power pool. As the Federal Energy Regulatory Commission has explained, tight power pools "extensively coordinate" their planning and operations, with central dispatch of generating plants. This coordination builds reliability--one of the long-term benefits, says the FERC, of a tight power pool.
Coordination also builds market power, however. And, as we all know from FERC Order 888, market power in transmission stands as "the single greatest impediment" to electricity competition.