Player characters are in many ways the diametric opposite of NPCs. Whereas NPCs are only important for as long as they are useful, and can be discarded on a whim thereafter, no PC can be left behind. No matter how logical it is in the story, no matter how strong a reason the character has for setting out on their own path in a direction different to the other PCs, it's unconscionable to actually have a PC not accompany the other PCs on their next adventure.

At least to anyone who is experienced in roleplaying games.

Transcript

Shmi: You must take Anakin instead of me.
Qui-Gon: But—
Shmi: I could never leave him here to the justice of Watto and the Hutts. Every moment would eat at my soul.
Anakin: But—
Shmi: You must go with Qui-Gon, Anakin.
Qui-Gon: But—
Anakin: I can't leave you!
Shmi: He will keep you, um... safe.
Qui-Gon: But... oh, yeah. I will.
Shmi: Anakin, listen to me. You're in great danger now. You must go with Qui-Gon.
Anakin: You're coming too, right?
Shmi: I can't. Qui-Gon has secured one slave's freedom. I must stay to make sure Watto won't seek you offworld.
Shmi: If we both go, he'll hunt us down and we'll never have a moment's safety. Now go, pack your things.
Anakin: Um... okay... {leaves}
Qui-Gon: But—
Shmi: Enough "buts".
Qui-Gon: But you're the PC!