Arguably, for a big chunk of GTK+ users (me included) it's not. I care about the work on the default theme as I can't really be bothered to install custom themes. For me, the same is true for Qt or whatever other toolkit: either they give me appropriate defaults or I will probably tend to ignore them.
That said, it seems that GTK+ developers agree with you more than you seem to expect: in the past few cycles they worked a lot on the theming engine, simplifying it to the point that the whole theme is defined only by CSS.
Instead of writing clunky, complex loadable modules in C, they can just mash together some CSS and customize every part of the UI. This is a massive improvement over GTK+2.
Of course, since the CSS can poke at the deep internals of the toolkit, complete stability cannot be provided: think of it as a user-level CSS that cannot cleanly cope when the target website changes.