Linearity isn't bad. One thing you'll notice is that nobody bitches about linearity in Prince of Persia: The Sands of Time, Uncharted, Gears of War, God of War, Call of Duty, Halo, Half-Life 2, Ico, Castlevania, Mega Man, Devil May Cry, Metal Gear Solid, Super Mario Bros, or any Final Fantasy prior to 10.
IE: Nobody complains about linearity when a game doesn't suck. When it does, though, gamers can and will ravenously glom onto anything they can in order to demean the product to the point of complete invalidity. Final Fantasy 13 was horrible, so both Final Fantasy and RPG fans in general responded by trying to make it philosophically flawed on a fundamental level in order to discourage the developer from ever making the same mistake again. The reality is that the RPG fans don't give two shits and a fuck about nonlinearity in their games, because they'll turn right around and play any number of games from the list I posted above quite happily minutes after posting their vitriol on a forum somewhere.
Even when games are nonlinear, they aren't about having a branching story or giving authorship to the player. Rather, the goal is to provide a sense of personalization, giving players a firmer connection with the game, its characters, and its story through their interaction and experiences with the game's world and associative identity. The goal is to provide a player with a means to explore the game's themes for themselves to better understand them.
Final Fantasy 13 keeps players pretty well in the dark on everything and is basically the second half of the game that Square was trying to make with the first half sloppily disseminated throughout as flashbacks. This is of course to cover up for the development nightmare that it really was and make it look like they knew what they were doing. The reality was that they were making the story up rather hastily as they went along, Crystal Tools was a piece of shit engine that they wasted boatloads of money on but doesn't have the functionality or versatility of something like Unreal 3, and their art asset pipeline is archaic and led them to produce a game that looks more like a launch title than a game that took 300 people and 4 years to make. Oh yes, their character models are splendid as usual. Have you looked at those environments' textures, though? Compare that to other games with a similar schedule, like God of War III. Compare it to ones with lesser schedules and fewer resources. Even Sonic Unleashed has a greater technical mastery of texturing and environment design than FF13.
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Sorry about that, every time I see mention of FF13 I just RAGE OUT on this stupid thing...
The point is that it's just not a very good game in ANY respect, and THAT'S why people complain about it.