Maybe it hasn’t been a problem for you, but in some parts of the country, it can be difficult to find providers for those with Medicaid-only coverage. Dual-eligibles get more robust reimbursement.
Additionally, there have been some excellent pilot programs in a number of states that seek to provide integrated care for the dual-eligible population, rolling in supports that aren’t normally covered by either insurance alone. This article doesn’t give the most elegantly-written explanation, but it links to a lot of sources: https://www.healthaffairs.org/content/forefront/expanding-home-and-community-based-services-dually-eligible-adults-behavioral-health
Bottom line, he still won’t pay anything out of pocket with Medi-Medi, and the providers will get paid better, and he may get some benefits he doesn’t get now, depending on what Maine does with duals.