I've never really found a good use case for reusing business logic, but just reusing types is pretty helpful. Although you can also get that via something like Swagger as well.
I too am more of a fan of the Swagger/OpenAPI approach (in general terms), but I find it valuable to be able to share validation logic too. (Unless your validation requirements are extremely simple.)
Of course Swagger/OpenAPI doesn't preclude using the same language on the client and server side, so one could always combine "isomorphic" approach with the "service definition" approach.
