It's linear in all manner of respects to usefulness.
There is nothing remarkable about choice if it's between chocolate or crap.
Acting as if X i some extraordinary display of choice and basically the best thing since sliced bread is laughable.
And annoying. But mostly just laughable.
In fact, I doubt I could say any bad thing about FFX without someone going about the game like it's some holy artifact that fell from the sky one day. On the same note, I probably couldn't say anything good about XIII without someone treating it like it rose out from Hell.
It doesn't really make your opinions too credible.
Actually allowing you control over every aspect of how you develop your characters is not something I see as useless at all, and in games it's especially useful because it means you can play the same game totally different ways each and every time you replay it. The fact that you can put Yuna down a different path like Auron's is one of the reasons I love the sphere grid system so much, because it allows experimentation and gives the player the ability to play the freaking game however they want to. You might assume something would be crap, but you won't know till you try it.
The sphere grid is the best levelling system I have ever played in any game, mostly because it does give you a massive amount of choice. You thinking it's linear just shows you don't even know the meaning of the word.
Trying to say something is linear, when it is completely the opposite by any definition shows how credible your opinions are on this subject.