Adam Clayton & Larry Mullen, JR - with all due respect to both, neither would have made a dent anywhere else if not for landing in a band with Bono and The Edge. To their credit, I suspect both know this, which is why U2 has never really had any real drama; they know their places and how lucky they are.
First example I thought of even before reading your post (although I was thinking Adam more than Larry, but I don't disagree with including Larry as well).
Jeff LeBar of Cinderella
Hmm. Interesting. I immediately thought of Fred Coury. But I suppose you are right that Jeff fits the bill too. Even though he really became an integral part of the band instantly, he really is easily replaceable. But then again, that is also partly due to the band's own internal dynamics after the split as well. I mean, it seems like it was always really Tom's band from the getgo. But when Michael Kelly Smith was still in the band before they recorded Night Songs, my understanding is that Tom and Michael shared the lead guitar spot, and that Michael may have even had the lion's share. After the split, Tom took it all. And I don't think that is a reflection of Jeff not being capable; he's a VERY good player, and has been from the beginning. That's just how it worked out in terms of band politics.
Mick Mars of Motley Crue
I disagree with that one. Mick has a very distinct style that is part of the band's signature sound. He may not be a flashy, charismatic player, but he has a very distinct style. Now, maybe the argument could be made that the other three members were the image and charisma of the band, and even if they ended up having a different player that would have taken the sound in a slightly different direction, that wouldn't have impacted their success. I don't think I could argue with that (kind of like CC in Poison, perhaps). But I'd be interested in hearing you expand on the idea.
Here's another one, going in a slightly different direction (and I know Cruithne already mentioned him, but I will second it): Ellefson. That isn't to diminish his talent or his contribution to Megadeth one bit. But Mustaine was talented and uniquely driven. He was going to be successful no matter what, or die trying. Whoever he got around him were basically going to be along for the ride. Ellefson was in the right place at the right time to jump onboard, and just being in Megadeth made him a name.
Another: Oz Fox in Stryper. Michael Sweet's vision for their sound and Robert Sweet's vision for their image are what made that band. Whoever they had accompanying them was always going to be secondary. Of course, it didn't hurt that Oz is a really talented vocalist who was really able to help shape the band's signature vocal harmonies, and that he committed to upping his guitar chops when needed. But I think the band could have easily had a revolving door in the second guitar department without anybody really noticing.