Good example.
you mean Jaime i assume?
if you told him to stay there you'd see him in Paris and he could get you Gunther's killphrase (or if you hadn't gotten Anna's and killed her the old fashioned way a Aug Upgrade Cannister i think)
Yes, I think Carter always leaves UNATCO regardless. As Shryke says if you ask JAime to stay and spy for you you get the killphrase; if you tell him to split he smuggles out an AUG cannister. In either case he meets JC in PAris and passes along the appropriate item. That's about the extent of player choice in the game.
Don't get me wrong, I think DX was fab, but still at it's heart it is a very linear first-person shooter. AN excellent one, of course.
minor nitpick, but if you tell Jaime to leave UNATCO you see him just before leaving Hong Kong - not Paris
(i've played it far too much)