@Gertjan
Well, in my case, we have sites to sites OpenVPN links.
Each site is a "vpn client" and there is an openvpn server in the middle.
Each sites have their own data server(s) and other equipments.
Users on each sites can access servers on other sites.
I believe NAT wouldn't work well in this case.
But, now, I think I understand your idea, it's when the client site only have "clients" users, then I understand your NAT suggestion.
Thanks again
Phil