Ok. Hypothetical. It's 1973 and you get the neat idea to record Arabella with Caballe. Do you a) ruin it by having Oliviera Miljakovic howl her way through Zdenka, or b) no, get someone good; it's 1973, maybe Helen Donath is sitting around somewhere singing with flawless beauty of tone and could use some cash.
B, you say? Well where the hell were you?