I've been wondering if Takatora deliberately picked a lockseed with a shield because it's indicative of his desire to defend humanity. Especially since if he wasn't using one of the two undersuits that have musou sabers, he'd be armed only with a shield.
(I'm also wondering if the reason Zangetsu has a musou saber is exactly that: Takatora deliberately picking something where he can only defend, and Ryouma going 'Wtf, no, have a katana-gun too, jfc'.)
The idea of it representing shame and dishonour works for me too, though.
Well, that could be a thing too, hell, they're not even mutually exclusive.
The fact that he DOES have the Sabre certainly adds a lovely contrast to Kouta in suit, with his defensive sword and shield vs. Kouta's offensive dual wielding.(Seriously, I've been thinking these two are supposed to be mirrors of each other since... I don't remember, but it was WAY back there. Like, before the Overlords at the least.)