I thought I'd miss customisation in Gnome more than I do now, a year later. If I didn't like Adwaita I'd probably still would.
But, eh, I can see the point of the developers believing in their design enough to give the option of changing it only to advanced users - who will have no problem installing Tweak Tool.
I agree with this. I feel as though it gives the developers added impetus to make the default experience as usable and polished as possible.