Yes, even if the OP's dad is on a lower-speed tier, "going D3" still has a number of advantages.
succinctly puts it (»Re: [Connectivity] Do high tier HSI plans bypass neighborhood co
said by EG:
There are benefits to be had by using a D3 capable modem on an upgraded to D3 system such as increased connection reliability due to the multiple bonded channel redundancy and failover capability. There are also increased capacity/reduced congestion benefits which result in increased reliability of receiving one's provisioned speeds especially during times of peak usage/demand. Higher PowerBoost assisted speeds have been noted as well. It's a win-win for both the provider and the subscriber.