I agree with Duane. If the datums shown are the mating features, the engineer can apply a call out to control the face back to -A-. So, yes the shaft axis may be used as datum -A-. Why did your engineer question the use of it as the primary datum? At first glance, seems like it would be the main function of the part.

Regards,

Geoff
Sr. CMM Programmer
Lapeer Metal Stamping
LMS Metrology Lab
Lapeer, MI