Fine, enough **** talking, if you want proof look at this:
http://www.bigprojects.net/sean/ssbmhitbox/ssbmhitbox.html
the yellow boxes are hurt boxes and the red boxes are hit boxes. Basically all aerial attacks have hit boxes that extend past the hurt boxes (ie. the red boxes extend past the yellow boxes), those are disjointed attacks. Sure, its true that things like swords generally are slightly more disjointed (its only slightly more disjointed, you can compare marth's nair with other nairs and the hit boxes extend past the hurt boxes about the same), but almost all aerials are disjointed. This is basically what determines how much priority any given move has (ie. moves with larger red areas that don't overlap yellow areas have more priority).
Brawl works exactly the same way, moves that have more priority have larger areas where the hit boxes do not overlap the hurt boxes.
Fair enough. I was under the impression that hitboxes and hurtboxes for physical attacks were linked in Brawl. Although if you look at most of them, the hitbox doesn't extend far enough beyond the hurtbox for it to really make too much difference. So let me re-word my previous statement:
Lucas's disjointed hitboxes extend beyond his hurtbox more than many other characters who are attacking using parts of their body. So in essence, unless he's faced with a particularly disjointed hitbox like one belonging to a weapon or energy burst, his hitboxes are more disjointed than a lot of others.
Looking at that frame data application, if Lucas and Sheik were to attack each other at the same time with Fair and Bair, they'd trade hits. If Sheik were to miscalculate her attack to whiff Lucas, then Lucas would win because his hitbox would pass over her hurtbox (her leg) without his own coming into contact with her hurtbox. Technically Lucas's is better as a hitbox since it extends much further beyond his hurtbox, but his legs aren't very long to begin with, so as an attack, it's not as effective.
But yeah, against characters with more range on their attacks overall, it will seem like he has low priority because he lacks the range that a lot of sword users do.
Since we don't have AR for Brawl yet, the best we can do to test it manually in slow training mode like I've been doing.
But to the point, Lucas's attacks actually do have decent priority. There's quite a bit of difference between his foot/hand and the spark from his attack. More than Dedede's Bair anyway.
I have to admit, Melee Samus's Bair is pretty crazy though. She doesn't even have hurtboxes on her feet. WTF?