A BT comes with a family who are often not BT.
Suppose the BT’s brother acts like a goy, eats like one and is marries to a man – that can create huge complexities. Do you attend the “simcha” of an intermarriage or a man “marrying” a man? DO you sit at a wedding that serves treif food, a chuppah officiated by a lady rabbi and a minister?
Likewise the BT’s parents can create a problem for the couple’s children when they do not act as a Zaidy or Bubby should act.
I agree with Health that BT’s should USUALLY marry BT’s and deal with common issues that are B”H foreign to FFB’s.