I think that the "hildonizing" of the widgets will be a lot of work.
If it is worth it, I don't know. I think WxPython is easier to program than PyGTK, and I believe it has more features/widgets than GTK.

I think the reason it isn't included by default is space. GTK is always included when you use Gnome, wxpython is optional. I doubt it will be a lot heavier to run, but I understand Nokia for not including it: it adds a lot more MB to the firmware.

Any GUI toolkit will need to be optimized for small touch screen use (part of the "hildonizing" is the choice Nokia made in optimizing the GUI). And that will be a lot of work, because of all the features of wxPython.