The system has no way of remembering NPC stats, does it? Reason I'm asking is that I have a LG campaign running with 5 PC's that picked up Meleny and Fruella and would like to complete the Wanted quest line in Verbobonc. Anyway I can drop Meleny and not have her be level 1 again when I grab her? And, yes ... my group was all about marrying who they could ... I'm a helpless romantic.
Most NPCs will remain how they are when you left them, but whether or not they will rejoin is another matter and is actually intentional behavior. They don't all just wait around for you to pick them back up, but some will.
My experience with NPCs (though with Ronald and Otis, not with Meleny or Fruella specifically) is that they return to their base position and retain all the XP & levels that they acquired with you.
Everyone knows you cannot get rid of Fruella short of her having an "accident" (Seriously guys, I know I am in no position to ask, having done 0 modding in my life, but can we NOT Mod that little stipulation away?!) and Meleny will ask you to return her to your home, I believe. BUT, everytime I have left Meleny outside her Dad's house in Homlett so that I can pick up another NPC (like the Prince) and then gone back to pick her up, she is still there, with all her levels, enhancements and gear. As stated before, other NPC's have specific things built into their character's AI about leaving nad rejoining. I believe some will only accompany you to the Temple for a couple of trips, or something like that - Otis and Mother Screng I believe are along those lines, but I believe NONE of them lose anything they gain while with you on your adventures, and if it IS in their AI to rejoin, they should be the level, and with the gear, you left them with.
I actually like that about her (Fruella). I usually have a LN Dwarf marry her and stick with it out of honor. If I ever have a CE character marry her, he usually just takes her out back after the ceremony and ... well, lets just say that he comes back with a nice, new Cleaver...
Oh I believe it is an EXCELLENT role-playing feature! And, as I mentioned, differerent NPCs have different paramaters about joning and leaving and such so it does fit right in with the game mechanics. I usually keep her on as well and she really does make a decent fighter. At this point her attitude has become endearing even! I guess it must really be.... ugh, can't say it, but must...Love! Still, modding in the option would be a goal if anyone is looking to make a simple improvement. Or at least modding out her jealousy statements when a different male character marries the sweet Meleny afterwards, because that really isn't in character. It shoudl be left, of course, if the same male character marries both, but not if Meleny marries a different man.