I always had just one skin option when running FA because it was too much work to keep track of everything. The mods were a lot harder to install and update on multiple skins with phpbb, also the services required template edits..
Now that we are past that, I can put up a selection. There are many people out there who feel strongly for dark skins and wont stay active on a forum that uses a light skin because they dont like the way it feels. Same works the other way around.
Personally I dont care either way. As long as I can read the posts and reply, thats fine. But not everyone is as easy to please as me.
I do like being able to change up the skin once in a while to make things feel fresh.