Lucina is for Marth players who are bad at spacing.
That's very rude and horribly misinformed.
Now back to OP's subject
Well since I already had to use this post, this is a completely unrealistic example but I think it might illustrate my point a bit.
Say that you have 100 hours to practice Smash 4 for a tournament, and imagine that it's true that the person with the most hours put into the game always wins.
Imagine that the average player that will attend this tournament has 55 hours of experience, with rare extremes of 120 and 20.
If you were a solo main, you would have 100 hours into your character and likely win against most people.
If you were a main with a secondary, let's say you have 65 hours into your main and 35 hours into your secondary. You might lose a bit more than the solo main but you at least have kind of an option to fall back to if you need it.
If you tried maining two characters, you'd have 50 hours into one main and 50 into the other, and now you're actually below average. You'd progress a few matches but you'd have trouble even with your average joe there,
and if you tried maining
three characters, you'd have 33 hours put into each, and now you're so below average it hardly matters you can play 3 characters. You'd probably hardly win anything.
Of course none of this works like exactly like that and this example is not perfect, but trying to give two characters or more similar amounts of attention is going to ultimately result in something not too dissimilar from that.
Truly maining more than one character is reserved for people who have played the game for a
long time. For most people, going this route means having a bunch of sub-par characters that might not be enough to really deal with anybody rather than one great character or one good character with backup(s).