"While I'm not with them in my own party I can sort of imagine that they're going about their own lives in a different location while also maintaining the player need to be able to call on them whenever needed"
Yes! That is the trick ... Narlen Darkwalk, Gorpel Hind, Soulefin, Drizzt to a degree, etc. Those characters were out doing things on their own, and occasionally their stories would cross paths with the PC. That makes them so much more interesting than NPC's who stand on a street corner like lampposts, just waiting for you to come back and report that you completed a quest. Those "active" NPC's were definitely a highlight of the BG series for me. Oh, and of course Shaellla, who really should have had more appearances.
