I don't discount dips, but I feel the decline bench is a lot easier to control and to load than wacking a heavy dumbbell around your waste and then hoisting yourself into position. There's a lot more to think about such as elbow postion, depth and the angle of the push, and eventhough it's easy to get into loose form on any exercise, with dips it's almost a certainty for most people.
Another problem with dips is that heavy people start out with quite a lot of weight (bodyweight) from their very first set, so warmups can be problematic for some.
I've always done a lot of dips in the past, but as I'm getting older I'm trying to save my joints from excessive stress i.e. train heavy in the safest possible way. I feel dips stress the shoulder joint a lot more than decline benches.