This flexibility is good. It allows you for example to create different versions of the same style, for different holidays. This way you can quickly switch to a Christmas, New Year, Halloween logo...
- your main style with normal logo (parent style)
-- same style with Christmas logo (child style)
-- same style with Halloween logo (child style)
- other style (sibling style)
-- child style of sibling
But you can even use a global logo for all styles:
- your main style with global logo (parent style)
-- other style but with the same logo (child style)
-- and another style with the same logo (child style)
So you see the possibilities are endless, whereas they would have been very limited in case of one global setting.