It's not so much a problem with Linearity in XIII so much as it is just atrocious level design.

And by that I mean my god is it bad level design.

And by not letting you back track to earlier areas as other FF have done wasn't really a bad Idea, as it played well into the plot of the game. It wouldn't make much sense if you could just up and waltz back to the ice plane area after all, not given the conditions.