To be fair, comparing something to something else is a valid way of evaluating something.

Any review that doesn't just rehash the game's features is subjective in some fashion. And that's really the point of a review. If reviews were only meant to restate information, you'd only ever need to read the ones with the most of it.

However, "the battle system sucks" or "the game is linear" are relatively meaningless without the author stating what point of reference the he or she is using. Saying "the battle system is a lot more fast-paced than the one in FFVII" or "the game doesn't offer as many opportunities for the player to take time off of the main storyline to do things like race chocobos or collect items like Materia" is an attempt to normalize what is still actually an opinion. It says, "when I say 'fast-paced,' I mean fast compared to this." It can be argued that such a review is far more useful for giving the reader an idea of whether they want to play a game, particularly if they've played the game the reviewer is comparing his or her subject to...

FFVII has likely been played by far more people than any of the other games and is widely considered the best in the franchise (by the far larger number of casual fans) so, as a point of reference to let the audience know where he or she is coming from, comparing a Final Fantasy game to FFVII makes sense...there's a good chance that the people thinking of playing the FFXIII you're reviewing have played FFVII at some point.

Now, if the actual point of the review was to demonstrate why FFVII is still the best FF by comparing it to something else--"OMG the final villain is a joke compared to SEPHIROTH!!!111one"--then it's not really attempting to be a review any more.