Why 10-pin and 6-pin ISP?
The reason for the GND pins is crosstalk. This is not much of a problem with short cables but becomes an issue with longer ones. The crosstalk is caused by the capacitance between the wires in the cable.
With the AVR, the original 'standard' was 10 pin by Kanda. Only later did the 6 pin emerge. All my AVR based boards used the 10 pin. A 6 pin connector can be 'made' by chopping up a larger one if the need arises.