For example, the main purpose of Chapel is to get rid of Curse cards which require the Witch. Er, no. No it isn't.
The main purpose of Chapel is to get rid of bad cards. All Curse cards are bad cards, but not all bad cards are Curse cards (hint: you start with 10 bad cards).
That said, the value of most (all?) cards vary based on the other cards available -- but "value varies" is quite different from "value is solely dependent on".