Lol. I read that wrong what you wrote about Zevran. Something along "There's not story-wise" reason for him to leave, romanced or not.
Eh, I honestly don't' see hardened Leliana leaving the Warden to join with someone she barely knows.
If Zevran is romanced then yeah, I don't think there's any story-driven reason for him to leave the Warden. He can probably do wham bam thank you ma'am freelance assassinations while staying on the move with the Warden, and I think by that point the Crows have decided to just stay clear of Zevran as long as he stays clear of them? I'm not sure. Either way, if he's romanced, he has no real reason to leave the Warden's side that I know of.
And if you're talking about Justinia, Leliana knew her even before she knew the Warden. If you mean the Inquisitor, Leliana didn't just hightail it back to the Warden because Justinia, a long time friend and very important person to her, was just murdered. Then there's also that gigantic hole in the sky. She stays because she wants to get to the bottom of it. Like Varric, she's not one to just ignore something like that and go, "Not my problem." Unless that's her personality when she's hardened. I wouldn't know because I don't think I ever did a playthrough of DA:O where she was hardened. lol
ALSO, it seems like it's the Warden who leaves the LIs behind anyway. It's like... "Bye, babe. You stay and do your thing. I gotta go find out if I can cure the Calling I won't eventually die from the taint. You know me. I totally won't get into any kind of trouble."