When? I can’t really think of any, outside of rather unreasonable cases and I suspect a few of the cases you might reply with could potentially be considered buggy behaviors in need of fixes, perhaps… 
I’m not sure it’s that big of a performance hit but then again there are still some people running blender on late 90’s hardware it would seem. More importantly if auto smooth really is that expensive I suspect it gets polled/reevaluated too often. Shouldn’t it only happen upon a change to the mesh, ideally only upon actions influencing surface normals/shading?
edit: I’ve stated this before and I’ll state it again, the real issue is that we don’t need 2 things to do the same thing, make/shade flat/smooth should be removed. Objects should be smooth by default and if you want edges sharp you can mark them explicitly sharp.
The automatic angle based edge/normal splitting could still remain a separate option however the reason we are all requesting auto-smooth be on by default is that edge split marks/tags only currently take effect with auto-smooth enabled, or the edge split modifier, but honestly that is yet another way of doing the same thing and I’d rather see the modifier removed to make space for more useful modifiers since one of the main reasons that has been given for not adding many new modifiers is that generally speaking the developers don’t want the list to grow too long(I’m sure theres more to it though). So I’d rather see the edge split modifier go if theres already another, potentially better way to handle/do this.