You hit the nail on the head IMO. X and XIII are both linear games, but the difference is that X doesn't feel linear, whereas in XIII you're literally walking through actual hallways for a large chunk of the game.

And you're absolutely right that linearity and openness are not inherently good or bad. My favorite FPS is the original Half-Life, and the majority of that game is a straight line. The most common complaint about Half-Life is that once you get to Xen, about 80% of the way through the game, the game drags and becomes much less fun. What else happens on Xen? The levels become much less linear and more open. That game was built to be linear, and when it stopped being as linear the game suffered for it.

Linearity vs openness is a choice that a game designer has to make. There is no wrong answer, but much of the game has to be built around that choice.