By the way, why is it that people want to play completely original NPC mods anyway?
Because variety is the spice of game replay life. And because, from a love interest angle, it'd be nice to let your PC develop a romantic relationship with someone isn't either A) an elven priestess with issues (now in your choice of alignments!) or, Anomen. Of course, given the quality of most mod NPCs, you're still not left with many options.
Also because it's interesting to see how various authors will handle certain events, namely Saradush or the Tree of Life, and the Slayer change, Bodhi abduction, and Master Wraith for a romance. And again, there are good original characters out there, if you can find them.
Plus I like seeing how authors develop their characters in the world of BG. I hated it when the Amber mod forced me to do a ton of errand quests, most of which seemed designed to enforce Amber's "I'm a tiefling, poor me, nobody likes me. Now go get my bracelet." In BG1 with Gavin, I felt that his quests should have had more time between them--I had to go to Beregost three times in 15 days. But after I finished those, I started to actually enjoy the mod. I liked that he interacted with the Ulgoth's Beard NPCs (he was born there, so it makes sense). If you're playing the romance, various members of his family show up, which gave a sense of him belonging in UB.
Next, interaction with other characters. The Keto/Yoshimo relationship was...well, unusual. I thought it was worth installing the mod. One of the few things I like about the Kivan mod is the lack of character bashing--he's on good terms with Cernd, Aerie, and LG/LN Anomen. Whereas I've Ctrl+Y'ed any NPC by Sillara, because those banters are horrific. A few of Kelsey's banters were entertaining, too. Perhaps the most unusual example is the NPC romance with Auren Aseph and Nalia. Again, certainly unusual, and definitely not canon, but interesting nonetheless.
There's also the variety aspect. I'll admit that I prefer the distribution of the Classic Adventures NPCs, but sometimes having the same group of characters all the way through can get boring. Plus I tend to play "glass cannon" characters, generally a kensai, mage, or archer, which limits my choice of NPCs. Since I mostly play good-aligned PCs and go for really high reputation, I can't use any of the vanilla evil characters, and since I hate spending 5 minutes prepping for battles, I tend to just hack/blast my way through. My tendency, then, is to use mod characters--e.g., Keto for magic missiles, Xan for emotion spells, or Sarah for ranged support. Or I could have Angelo cast stoneskin and use him as a tank character, or have Auren hack her way through a battle. So original NPCs have a tactical aspect as well.
Last is humor. I have several joke mods installed at present. Skooter is definitely an original NPC, despite the utter stupidity of the content, and I don't plan to uninstall him. There are some pretty good lines in other mods. Before the novelty wore off, Keto's tendency to wake up with a hangover was amusing. Getting Xan drunk was hilarious. Angelo had some truly wonderful lines. In Classic Adventures, there's an engineer you have to kill. His name? Scotty. Or the "Encounter of Serpentine Nature in the Pocket Dimension." Funny, but the mod is hardly a triumph of writing. I still play it, though.
My point here is that original mods/characters add another dimension to gameplay. The problem is when said dimension is not worth the download time.
Incidentally, I discovered several very interesting things about mod Kivan in BG1 NPC
1) In versions 17 and before, it was possible to have a romance of sorts in BG1. It was removed for version 18, but for some reason, the romance conflict with Coran is still there.
2) Deheriana is supposed to have died fifteen years ago. IMO, this doesn't account for Kivan's obsession. I suspect that it's to prevent anyone playing the BG2 romance from feeling guilty.
3) Her name is mentioned sixty-four times in just one dialogue file. Granted, a lot of it is because lines are repeated, but still...sixty-four times...