It would not be the first time Sakurai contradicts himself. For instance, read the first paragraph, after that... just laugh at the self-referential incoherence.
In the end, if a game isn't fun, then there's no point. Of course, it would be really easy to make a game by churning out a ton of similar characters, but that's not how I produce games.
Initially, I had considered including Lucina as one of Marth's alternate costumes. After all, she has a close relationship with him in FE:A. In such cases, even if two characters' names and voices differ, as long as they function the same way, I assign them as alternate costumes. The Wii Fit Trainers, Villagers, and Robin are examples of this setup.
However, even though Lucina shares her physical stats and techniques with Marth, the characteristics of their attacks differ. When two such similar characters function in an even slightly different manner, I give them a separate spot on the roster since that will affect battle records and whatnot. In that sense, you could say she was very lucky to join the fray!