When discussing XIII elsewhere and some of its more irritating gameplay aspects, the topic of the frequent and nonsensical party split ups or limits was addressed. Another poster pointed out IX really wasn't any different.
And having just replayed it, I see his point. You use Steiner for, what, 10% of the first two disks? Maybe 15?
I WILL ESCAPE WITH YOU ALL!
NO WAIT, I WILL GO BACK WITH BEATRIX AND FREYA BECAUSE REASONS!
You are actually short a full party for the last half of Disk 1/first quarter of disk 2 unless you know about Quina. Speaking of him/her, I love how s/he just randomly leaves your party for the rest of Disk 2 once you get to the Mountain Path even though s/he's right smurfing there in Madain Sari.
People complain about how long it took XIII to "open up" or "give me complete control of my team." IX is no better in this regard.