But exactly; the top screenshot is from Risen, which used an older version '08/'09 of SpeedTree,
http://www.speedtree.com/gallery/images/...f48c55.jpg
For tree's closer to the camera, its better to have lots of geometry/quads/etc. In later versions of SpeedTree, they use shaders to fade between the different LOD's. Billboards are only used at middle-LOD stages; copying this hybrid-approach would be nice (and at the final stage having a pure sliced billboard imposter + additional horizontal slice in the canopy), but i'm still against pure billboard implementations.
My vote goes to this (with current technology) pipeline:
High Poly Geometry Tree (< 15,000 triangles) <-> Billboard+Geometry Leaves (< 7,000 triangles) <-> Billboard Leaves (< 2,000 triangles) <-> Billboard Tree + Horizontal Slice (< 100 triangles)
To automate/tweak these approximates as much as possible would be great.
Edit: E.G. Some ideas, have an in-engine "leaf spray painter" - setup the textures and it manages all vertex shaders, billboard LOD's and animations for you. You only supply the trunk models, but be able to setup PhysX destruction/bending/breaking joints on them.