While I agree the monster system is not half bad and this is technically a swing at the proverbial 'low hanging fruit' I have to say I think it works to XIII-2's detriment.
The power system of monsters is unbalanced. They start off pretty weak, very quickly become far more powerful than your MCs for about 80% of the game; temporarily dip down into 'slightly weaker' once you start hitting Lv. 99 in most of your classes, then become vastly stronger again once you start utilizing infusions. There is no sense of progression or balance at all; it's pretty much a grab bag of mixed random messes. Honestly it's a small miracle it didn't turn out as a steaming pile rather than the 'fairly good' it is.
Secondly, as a personal note, I typically hate it when developers force 'You must use the monster' systems into their games. If I want to play with monsters I'll go play Pokemon. Additionally it feels like a crutch so the writers don't have to include more characters; which might be a good excuse if Serah and Noel were these 'deep, story-intensive, study them at a college literary course' characters. But they're not. And there are plenty of opportunities for a third human to tag along with the group. Any of the four from the starting village for example; or even Alyssa. Hell, even Snow and Hope could count.
They already travel through time. And the whole 'but if they don't do X and Y instead of traveling with you it creates a plot hole' doesn't work because it's a time traveling game. I.E; pick up Hope at 12:52.0 p.m in 100 AF; you can visit him at all the plot relevant points at future dates since at the very end of the game, right before all the time gates close, you send him back for one last trip all the way back to 100 AF at 12:52.1 pm. Time loop closed.
Lazy, lazy writers. :-P