Personally, considering that addle is mostly used (as a voidwatch proc~) whenever the mob is unable to be silenced, I'm surprised they didn't consider buffing it to actively penalize the target if it does cast spells. Something like:

Addle: Decreases enemies magical accuracy and increases spellcasting time. Saboteur: Target takes damage whenever a successful spell is cast.

Or make it so that spells cast whilst under the effect of Saboteur'd Addle take twice as long to recharge.

Why even stop there, change the all the current debuffs to have an extra more potent effect whilst under Saboteur? (and then half the recast timer on Saboteur)

Slow : Targets job abilities and weapon skills take twice as long to ready. RDM gains a fast cast bonus depending on strength of targets slow.

Paralyze : The next weapon-skill used is automatically paralyzed (although, I'd imagine just interrupted would suffice) and is disabled for the next X+ weaponskills. (Only 1 weaponskill maximum can be disabled at a time)

Blind : Enemies nearby take +X damage whenever target afflicted misses.

Dispel : Removes an additional buff. Buffs cast upon target wear off 250-500% faster.