There are only a few exceptions:
OG!Hector: Already have the LA!Hector and L!Hector with good IV
W!Tharja: Too much good fodder, and I don't like her (H!Henry is my main armored mage)
Ofc, always do whatever is best four your goals in your current situation. But foddering off Eir for swift sparrow will seem crazy for char collectors.
Well, right now it's one orb and in the long run a couple more, but probably not a lot more than the cost of sniping for swift sparrow until you get it.