As far as directly viewed photographs are concerned yes the textures are often "off", however they are designed to work with lighting in 3D applications and so may be intentionally tuned in terms of levels, contrast, etc. for best results in these situations. Certainly you may want to tune your textures for your preferred result, but just be aware that there is not necessarily a "right" way to do it. Unfortunately since these textures are photo-based they will inevitably have some lighting and color information that is not "neutral" and so may need to be manually adjusted to fit certain scenes. If the "essence" of a leaf texture could be generated truly procedurally it could be more "idealized" and neutral and thus fit into scenes more naturally since it would be directly responding to the scene lighting, etc.
- Oshyan