The companions that saw the most use in our games was RMCI, RMCII and RMCIV. From the first, the background options and the smooth stat bonuses were by far the most used section. We never used Arcane magic, and I really detest semi-spell-user for professions that are perfectly legit non spell users (i.e. nightblade, why is a thief casting magic?).
RMCII was a must have because of the comprehensive skill list. That though was probably where most players felt that RM was too complicated, but our group loved all the skills that we could develop in.
RMCIV was used for A) the point-based background options and B) the smooth spell list advancement (which made it into RMC, nice), and C) the one-roll-spell option.
The other companions were always fun to read, but we almost never used them. *Occassionally* we'd find a useful profession, but we were non-spell-user intensive and most of the companions provided spell-user professions which never fit into our campaign. I liked the bounty-hunter (non-spell-user ranger) and I think we created an assassin once, but the rest we never used. I did like the create-your-own-profession rules from the ArmsCompanion, but by the time I got that suppliment, our gaming days were fizziling out.
So, I'd *love* to see extra suppliments for RMC, if anything to pad out my collection. They were always fun to buy and just read, even if they were never used. As Ranger said, all the Companions were optional, and we simply took what made sense for our campaign. BUT having the option *available* helped us come up with good ideas for later campaigns.
-shnar