A few more utility functions and hooks in the rendering pipeline would tremendously increase customizability and allow for extensions doing some more advanced things like area light soft shadows, reflective shadowmaps, screen space photon mapping, ...). Even if they were easy to break the rendering pipeline, this would open up new areas for customization.
Camera.OnRenderedPrepassBase() -> extend the standard pipeline with custom steps/passes
Camera.OnRenderedPrepassLighting() -> extend the standard pipeline with custom steps/passes
Camera.RenderWithPass(...) -> piece together custom pipelines, customize shadowmap rendering
Light.UserData -> a bit like a material, to pass user data to shadowmap rendering and deferred rendering of the light
Light.OnShadowmapPreRender() -> set custom shader variables per light, customize shadowmap rendering
Light.OnPrepassPreRender() -> set custom shader variables per light, customize light rendering
or/and hooks to replace built in parts of the pipeline
Camera.RenderPrePassBase() -> replace built in
Camera.RenderPrePassLighting() -> replace built in
Camera.RenderPrePassFinal() -> replace built in
Camera.RenderForwardPass() -> replace built in
Light.RenderShadowmap() -> replace built in
Right now (Dec. 2012) there are two choices for customized rendering - no middle ground: doing minor customizations within the Unity pipeline or completely replacing it with a custom solution loosing some compatibility and convenience. So you do not see any big customizations to the rendering pipeline in the Asset Store and elsewhere.