One reason could be that you still have an infrastructure if your Windows Server fails. But you probably have more important problems without the Windows DHCP than your APs.
Still, for some customers, I put the whole management network for all Meraki Devices on a VLAN that terminates on the firewall. That again increases the likelihood that the infrastructure works regardless of internal failures.
Only for guest type networks that you want segmented. This way you don't have any concern about this network having access to the server VLAN. Other then that. I prefer Windows DHPC server. It has more options and easier to configure.