You don't truly grasp what sour spots/sweetspots do for characters...But compared to Marth, what's the difference? Even though Marth has 2 hitboxs, the weak one rarely adds anything and is usually discouraged. It may be useful for comboing off of nair/bair, but mostly it's just a worse hitbox for Marth in every way. So essencially, Marth really only has 1 hitbox you want, just like Lucina, but unlike her, also has to worry about an inferior hitbox. I know that Marth is still better because his tipper hitbox is better than Lucina's, however it seems unfair to say Marth has more depth when his weak hitbox doesn't add anything.
They also have arcing hitboxes, this means that in a purely horizontal or vertical situation (or whatever rotation), untippered hitboxes can be going further than the tippered ones (this is something you usually get people to master with forward air, as it's mostly the only applicable one).
And funnily enough, Marth sour spotting in this game is actually something I encourage. It's required for him to combo well, which he is actually quite not-terrible at, and the fact of the matter is sour fair can lead into tipper fsmash (as can sour up air/bair). He's seeing follow ups from his sourspots for twice to thrice the range Lucina is able to get the same combos. Furthermore most moves the two have send in a Sakurai Angle, meaning at a certain point is the difference between a semi-spike down (which forces a tech chase) vs sending 45 degrees which doesn't give those opportunities.
Lucina really only wants to tipper because all she wants out of her sword is maximum safety which she gets less of than Marth does. Or to put it the way Sakurai does "doesn't have to think about where she hits with in her sword", because she doesn't get anything for the alternatives, ever.
Marth isn't punished (in design) for not tipping, he's rewarded for spacing closer when he wants to space closer.
This is something that is intrinsically different between Marth in both Brawl and Melee (in Brawl there weren't combos, and in Melee his tippered actions gave better rewards for combos/etc), and I guess is a nice touch were it not for Marth being terrible.
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