Is it useful to enhance security by implementing RODC in the same site, which RWDC exist also? Pointing the site user the RODC instead of the RWDC.
Thanks.
I wouldn't consider implementing a RODC in the same site just so that users are primarily accessing a read-only DC, since all write operations will just be sent to the RWDC. From what I've read, it's best used when:
Yes, it is useful ... having a RODC on the same site is useful when you have to power down your RWDC for maintenance.
A less common but still useful deployment is when you have application servers located in a DMZ that need read access to your internal Active Directory services. There's a few different security models but one involves extending your forest into the DMZ by placing an RODC there.
See: Active Directory Domain Services in the Perimeter Network
Placing RWDC in a site with RODC makes RODC's security features useless. Please read this: https://technet.microsoft.com/en-us/library/ee522995(v=ws.10).aspx#bkmk_placinganrwdcinasitewithanrodc