Tilt: Okay, so you save Kid, and Mel joins your party, right?
WRONG!
No, you have to wait until after you become Serge again. >_< And she isn't even worth it after that. She's why I always pick to not save Kid.
Incorrect.
You don't have to wait until you become Serge again to recruit her.
Granted, she is one of the characters that becomes available almost near the end of the first part of the game, but of course you can use her during the time before you become Lynx.
Combat Usefulness: Her only possible saving grace is the fact that she can steal, but so can two other characters. She does have a double-tech with Kid when it comes to stealing, though.
Yes. Her only real useful purpose is the fact that she can steal and that she's one of the few characters that actually has a Double Tech. And its with one of the main characters of the game, mind you.
Now, considering the fact that Kid is available during the entire (well, almost) first part of the game, when she becomes available for recruitment it's already too late, because Kid has already recovered no matter which plot path/branch you took.
Regarding her double tech, I really doubt that there is
any real use for it at all; considering the fact the it might help increase the succes rate of rare items is practically void. And I am fairly cartain that it doesn't, testing it out thoroughly in the fight with Harle on Hermit's Hideaway to get a pair of Moonglasses.