Diffuse bounces are very misunderstood. So are the bounces in general. Before you can explain bounces, you have to understand what they actually are and how they are calculated.

The camera sends out a ray and then it hits something for the first time, that is not a bounce. That only starts the bounce counters, no matter what it hits.

Consider this example.

Lets go over some possible bounce scenarios with these settings.

Camera, diffuse (start counters on next hit), glossy (1st bounce, glossy), diffuse (2nd bounce, diffuse). Max of one diffuse reached, terminate ray. Ray cancelled in two bounces

Camera, glossy (start counters on next hit), transparent (14 times/bounces), Transparency can override max bounces, and is the only render setting that can, glossy (15th bounce), diffuse (terminate ray, max diffuse reached) Ray cancelled in 16 bounces, which had nothing to due with the 16 max trans bounces, or the Max Bounce setting because of the transparency over ride.

Camera, glossy, glossy, glossy, diffuse, glossy, glossy, transmission, transmission, transmission (terminate ray, reached 8 Max Bounces)

Camera, diffuse (start counters on next hit), 16 transparency, (terminate ray, max trans reached). Which is where black spots in trans hair come from if the Max Transparency setting is to low.

Render setting have more to do with lighting and materials in the scene than anything. As you can see from the examples, the numbers in the bounce settings is conditional to what it is going to hit in the scene.

If you have no specular and reflection in the scene, say a cartoon render, glossy will have virtually no effect on the render at all. Diffuse is going to make a huge difference, depending on the lighting.

But if that scenario is reversed, and glossy is set to low with mirrors in the scene, some things may not show correctly in the render.

The best settings are really the minimum you actually need to properly render the scene, based on the scene.

And that is different, for every scene.

On top of these settings, you also have the other render settings that can massively affect the information and direction of each bounce. Refractive and Reflective Caustics, Filter Glossy, and Clamp sample settings. 

If you are creative with the light path node, you can add far more conditions to the actual shaders themselves. An example of this is terminating a max transparency termination to transparent instead of the default of zero (black).

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