I think the move definitely makes sense for Sony. PS3 provides a catalyst for them to build out this fab capacity, with some strong assurance of cost coverage in the future, but Cell and RSX production isn't the end-all be-all. Now having the new Nagasaki line in place, this is a fab that will be useful to them 10 years out, should Cell succeed or even should it fail. They'll always need chips, for PS4 (assuming there is one, right?), for their various CE devices (insourcing IC chip R&D and production is a new push for their CE division), and for any other of their own chips which they sell to others - digital camera chips being one strong one for them at the moment.
So I think the PS3/Cell provided the critical mass for the build-up, but now since reached, win or lose on Cell this fab capacity is going to be useful to them.
Remember that CE is a very high-volume field, and fab capacity gets soaked up if you're doing things right. The GPU comparisson has been made, but GPU's truly are a business more suited to outsourcing. NVidia and ATI simply do not have the capital for a modern fab investment; for them the risk would be enormous. With product cycles as quick they are, better to have the flexibility to jump from fab to fab should a decisive advantage present itself, or to contract out to multiple fabs should volume warrant it - all on a case by case basis.