This is hard to answer... I think it is important - and it is not.
Look back to the N64 days: N64 cartidges had about 64mb storage while a CD was at 650mb which is about 10 times the space.
Still games like Zelda 64 were just as big as 32mb (!!) and they still looked gorgeus.
So Blu Ray is not necesary but it can make developement a lot easier if you´ve taken the hurdles of the PS3´s architecture.
(I guess this post was full of spelling mistakes... )