X was linear in storytelling, which is not a bad thing. XIII was physically linear, as in dungeons and areas are all long featureless straight lines.
The battle system being good is one of those things where it doesn't really matter at all, I think. If you think any Final Fantasy game is good particularly because of the battle system, I don't think you're really paying attention to what makes these games go from good to great.
That being said, FFXIII's battle system is pretty good; it just doesn't make up for how pretentious and boring the rest of the game is. Even the music, arguably the most popular part of the series, kind of just straight up sucks.