The main reason to allow players to let them establish the relationships is for incest stuff. If you don't really support that in the writing it serves no purpose at all, and even the current default "friend" is often out of place and unnecessarily capitalized.
That being said, establishing a handful of relationships at the start is very annoying when you know nothing about the characters. Either create a relationship panel where you can change them whenever you want, or have it come up naturally when you first encounter the character. The first few minutes are very important and set the tone for the rest of the game, don't put grunt work there.