They're not entirely neglected. Its just an issue of what's of greater worth. Geometry shaders in DX and the relevant OGL extensions make it easier to make fins. GPU Gems 3, "Next Generation Speedtree Rendering" has a chunk devoted to using fins and displacement maps to add irregular silhouettes to their trees. Probably would be a good article to dig up.
Discarding the fragment could "cut" into geometry without using fins though. I suppose something like a grayscale SSS density map could be used to represent the frequency at which a pixel will be cut as the dot-product of a normal map (post adjustment against the surface the normal) and the view vector approaches 1.0f. The geometry of the object would be an issue though, don't want to cut holes into the center of it because something is sticking out there.
This can also give you some ideas:http://citeseer.ist.psu.edu/menezesdeoliveiranet00relief.html