You don't have to use legacy as a surname, you can have "Mr Chiss, the Sith Pureblood legacy" for example. That shows they are linked via legacy, but not family.
But yes, I agree and think Legacy was a stupid implimentation.
True...but I see that option always there, and logging into any new character and the last name is there. And if I don't use it, then my character doesn't have a last name except in my head, and it's not like there's a bio option for your character
Though, I find the legacy system fun, as I went about trying to figure out how I wanted to make the family tree work, trying to combine in the storylines with it in some fashion.
I do wish you can display your companions on the Legacy tree when/if you get to the "Lets get married!" option.