Several reasons.
1. A ground inerspersed with a signal is less prone to noise and adjacent signal coupling.
2.10 pin IDC connectors and cable are more easy to come by.
3. If you don't want to use IDC cable and headers you only wire discretely what you want.
4. You have the flexability to share one 5VDC regulator.
5. 5VDC might be needed to support opto isolation.
Regarding pins 1 and 9. Two of the boards, the 3977 and L297-8 use pin 1 for enable. The pad gives the flexability to wire as you see fit. Pin 9 can be arranged to give or get 5vdc to those boards also as well as the discrete unipolar.
Phil |