That is personal taste. What makes Viconia more important than Mazzy or Jan? I personally have Jan and Mazzy always in my party but do not care for Viconia at all.
I would have been more mad, if they brought in Jan or Mazzy and not get them right.
I'm not saying they are more important than Viconia or Sarevok, but from a players perspective neither are less important.
"Important" is not a word I would use - more like "defined".
Amount of narrative content each companions has varies a lot - with Jaheira being far ahead of everyone else. Viconia by becoming a romancable option, became more defined in BG2 than a lot of "lesser" party members (lesser in terms of content). Yes, they absolutely could get Jan and Mazzy wrong. But I think it also would be easier to do something off-beat with Jan and Mazzy that wouldn't feel so off because, we really didn't get to know either of them too well. Not that one should bring familiar faces just for a sake of it.
Viconia and Sarevok are just such bizzare cases - I wonder how they came to be. If you would tell me they replaced different NPCs in the last stretch of the game I would believe it. It just doesn't feel like the scenarios they find themselves in have been crafted with them in mind. It just doesn't make sense for them to be there, or be... like this.
