I too really like advantage/disadvantage systems for how they can help differentiate characters. Homogeneity (particularly at higher levels, as beaucoup skill-checks have raised even rarely-used skills to high levels, and rendered the Brawling Fighter very stealthy, the Wiry Thief into a deadly fighter, etc) is a bane of BRP, I think; so Adv/Disadv mechanics could be a welcome palliative. Also, more hooks for characterisation/RP, character-building/backstory, etc... these are all welcome, and these systems excel in such efforts.
HOWEVER, I am reluctant to delve into this area because of playbalance issues, increased chargen-time, abuse by MinMax'ers, bloat/creep, and other points (as raised across this thread).
That said, I've got some ideas percolating for a BRP campaign, and (if the brew ever gets served (likely a year or more)) might consult some of the mechanics referenced above to create a small-ish system of Adv/Disadv available at chargen... so, thanks all for the discussion!