Zig Zagged: If Marcy doesn't get to take the gear the player bought her along, her mission fails and she's horribly killed. However, some players consider Marcy The Scrappy and would gladly see her killed... but doing so locks out the Golden Ending. On the other hand, stripping Guest Star Party Member Kyle of all his gear and sending him off without a scrap on him doesn't have any effect on the plot...
Nobody ever leaves the party after joining, even temporarily.
Alternately, anyone who leaves the party always leaves their equipment behind.
The Guest Star Party Member lacks a usable inventory so that players can't give him any items, the better not to lose them when he leaves.
Enforced: The game tries to create the impression that each party member has her own individual motivations, and that teaming up is just their way of furthering them. Obviously, any gear they earn in the process they're going to keep for future adventures.
Lampshaded: "Somehow, I knew she'd abandon us as soon as we geared her up like that."
Invoked: Marcy knows she has to leave her friends for a while to do something really important, but doesn't tell them beforehand because she knows Dirk's Jerkass friend Sten would force her to leave all her expensive equipment behind... and that's suicidal when moving through enemy territory.
Defied: Dirk refuses to let anyone leave, because it's too dangerous to travel through The Empire alone.
Discussed: When Marcy decides to leave, she gives advance warning to Dirk, so that they can split the gear in a way that keeps them both safe.
Conversed: "Agh! Don't you hate it when party members in these games leave suddenly and take all the expensive equipment you just got them?"
Aware that Marcy is probably leaving soon, Dirk makes her strip off everything he's bought for her. This leads to Marcy getting overpowered by The Empire's soldiers and killed before she could afford her own replacement equipment. Horrified, White Magician Girl Luna calls him on this: What the Hell, Hero?? You Bastard!
Alternately (or perhaps also), the game has a Switching P.O.V., and may temporarily put you in charge of Marcy at any point after she's left the party proper. If you left her gear on, the next part will be a cakewalk. Otherwise, you've just crippled yourself, big-time.
While playing as Marcy, you find a store that sells better equipment than the stuff you gave her to start with, but since Marcy didn't take any money with her, you'll have to either grind like hell or sell your old equipment in order to afford the new stuff.
Played For Drama: When she gets back, Marcy and Sten get into a huge argument about her leaving with all her equipment, until they abruptly turn to Dirk with a choice: let Marcy back into the group (and watch Sten storm off in anger), or bar her from rejoining? (The best choice, of course, is siding with Luna and defusing the argument entirely.)