Do we know a single person that have different logos in the different
spaces of the same wiki?
I've never met any, but I don't know all users.
To me, it just creates confusion for 99% of users, for the sake of helping
1% of them (supposing they know how to do it).
That would be ok if that 1% were contributors to XWiki, but it does not
seem to be the situation.
My proposal is to have a single input, for the logo of the current wiki
(subwikis can fallback to the main one), et trash all the other options.
My 2 cents,
Guillaume