The client supports a Primary and a Secondary registrar. You want to offer customer as much High Availability as possible. When you have a Pool Pairing setup and an SBA on site you have to choose between voice resiliency and have no backup for conferencing or go for the pool pairing with WAN redundancy.
Have a third registrar in the client would be able to make use for SBA and both Pools in Pool Pairing topology. The site would benefit from all failover scenario's.