Yeah, that is true. Street Fighter creates their own characters, and gives each of those characters a reason why they have their movesets. For example, Gouken is Ken and Ryu's teacher, Ken and Ryu studied together, hence the similarities in fighting styles, etc.
I could say the same about Lucina though: she's Marth's descendant and has his D.N.A. She even has the same sword that defeated the Shadow Dragon, even though it's design changed over the years. Maybe that's why the power isn't focused on the tip anymore. It's no wonder why she would play similarly, though, with both being part of the same royal family.
About the nerf thing you said:
- We don't know whether Lucina > Marth, nor Marth > Lucina. Lucina might be easier to use, but in no way does that mean she's a better character. I think it's wiser to wait for the release and test the characters rather than jump to conclusions.
- Street Fighter nerfs and buffs characters throughout games interchangeably. Ken just got buffed in USF4, while Ryu is pretty much in the same tier as him now (some think Ryu's slightly inferior than Ken, but that's debatable). On the other hand, Sakura (another Ryu "shoto" I forgot to mention in my previous post) used to be considered much better than both of these until USF4's nerf.
In the end though, I don't think you're wrong to think that Lucina is a waste of a slot: she does have the same moveset as another character with slightly different attributes, and I understand why anyone would be upset to see the same moveset again with slight tinkers.
Personally, I'm indifferent to Lucina being in the game, as I've never enjoyed playing as Marth, Roy, and I probably won't enjoy Lucina either. It's not necessarily bad to have a "simpler" version of the same character, though, and - given previous history - I'm positive that Marth and Lucina will end up playing and performing differently.