it depends guys, if you look as Mass Effect 2 that was on 2 DVDs, you would have to switch the disks once in the middle, then again near the end. theres nothing to suggest Square couldnt do that, despite the huge size of the data.
unless for some reason they couldnt because of the size of the data; personally i think FF13 was designed to be a linear game, there was never any intention to send the player backtracking except for Gran Pluse.
think about it, those areas were as you both said 'straight lines' anyway, and after youve picked the items in that area, you never really need to return to them - the game didnt seem to be geared for exploration like FF12.
there was no reason for backtracking, which is why the linear dungeons were so long imo (they were very long). anyway my problem was not with linearity, it was the lack of anything meaningful happening in those dungeons, but thats another story althogether!!
