(This perspective doesn't really surprise me considering they run a "Biblical fanfiction" blog. Heee!)
It's an intriguing idea, first off--especially if your character doesn't have much of a center and seems to be walking around either getting pulled to their destinations by plot-based puppet strings or wandering aimlessly. And those old sins and virtues are compelling for readers because they're embedded in our culture's collective consciousness; we recognize something ancient and important about these themes when we see them pop up in literature, and we find ourselves drawn to them.
This tendency for readers to be attracted to the familiar is part of why I chose to tackle a fairy tale retelling; readers would come in with a connection to my story, and then I could make them love the old tale in a whole new way. I adore archetypal symbols and incorporating them into my work; for instance, in my novel Bad Fairy, there's a huge emphasis on the four elements as an influence in the characters' life and work. (On my main website, I even have a What's Your Element? Quiz that ties into the book. Take it and find your element, it's fun!)
So I think if an author decides to use these sins and virtues to give some focus to their characters, they really need to plant that seed in the center and then feed in a variety of plant food so that this sin and this virtue will get distributed throughout the flower's roots, stem, leaves, and petals . . . mixed up with everything else that goes into making that thing bloom. And don't ignore the rest of the garden. Those other plants--all with their own center seeds, all with their own unique root systems the viewer can't see on the surface, all with their own ways of enduring their natural circumstances--they will all have an effect on your garden's centerpiece, and as long as you recognize this, growing a character from such a powerful kernel could be a fantastic idea.
Regardless of whether you pick defining sins and virtues for your characters, you might enjoy--just as an exercise--going through them and thinking about your most important characters' relationship with each one. In Bad Fairy, my protagonist's greatest "virtue" (of those considered "heavenly") would definitely be her diligence, and I think her greatest "sin" would be her pride. I say diligence because she's driven by an obsessive perfectionist streak to achieve and triumph, and there is a secondary "virtue" of charity filtering in there because she ultimately wants to use her accomplishments to help others, as well as a wisp of kindness because she genuinely cares enough about her friends to put herself out and make sure they don't get left behind. But the pride ends up being her downfall--she reaches so fiercely for superiority and recognition that she is scorned for not knowing her place, and this leads to wrath against the people who roadblocked her progress and envy toward those who got what she believes she deserved.
None of those virtues or vices define her, though. I don't think I would have developed her as such complicated person if I had started by focusing on how many different ways I could express just the most obvious pair (diligence and pride). I'd recommend that people who want to use this technique should feel free to celebrate a character's core traits, but should be careful about using them to limit that character's expression. For all their inspiration, sins and virtues are concepts . . . and they shouldn't be mistaken for people.