The reasons why Vince and Steph will never allow Heyman, Cornette, and Bischoff to help out with booking/writing of WWE shows:
• Vince would never allow any of them to do so because of his ego. Hell, he spent the majority of the last ten years trying to put Bischoff out of business. They're just kept around so they can't be useful to anyone else who is trying to start/run a promotion. If this is the case, it's rightfully so since he's the only one in business today. However, he is propably paying them very well and should definitely try to find a better use for them to justify keeping them on WWE's payroll.
• Steph doesn't want anything to do with them because it's quiet obvious that they obvoiusly understand the nuances of booking a wrestling promotion more than she does. Bringing them in would propably make her look foolish (or make others question her ideas, at worst), and as the head of WWE's creative team she would be looked down on, especially if any of the three came up with any ideas that actually drew. Not only would Steph look less important in the WWE machine, but she would certainly look foolish to her father because of all the ratings losses during her time in command.