I'm not even sure how great dual injection is at preventing valve deposits, since it's only used at low loads and only a small amount of fuel is actually sprayed by the port injector when they are in use, in the Toyota implementation , I'm not sure how everyone else is doing it, the primary purpose is to create a fully atomized lean pocket in the entire cylider and then when the in cylinder injector injects the greater volume of fuel you get a rich fuel mixture in the middle of the cylinder near the spark plug and a lean pocket further out, which apparently helps keep the catalyst up to temp and I guess reduce fuel usage too or something.