Yeah you still need to be able to configure learned debuffs on/off. And when the author learns of a debuff that should be default off (raidwide aoe shit like you talked about), it's still possible to push out a version with that debuff defaulting to off.
I'm just thinking that the autolearning feature would make it so much ... well.. userfriendlier for those of us being early into new encounters =)
Couldn't 99% of the stuff be autolearned by just examining who casts a buff? Check targets / mouseovers - if it's a boss level mob, learn its name. Then when you scan buffs, check if it's cast by a known boss.
You could of course still add new buffs to the list of configurable buffs so that people can turn them on/off.