In those situations it doesn't really matter much. The problem is that there are far too many buffs/ debuffs that a target will likely have all the time and no suitable way to filter it. Mostly it doesn't matter since things either stack (every DOT effect), over-write with the most recent (most debuffs), or are irrelevant (most enfeeblements like slow, heavy, silence, etc.). Filtering to only show your own debuffs would work for most people most of the time, but there are situations where, for example, a WAR or NIN would want to be aware of the other one using their slashing resistance debuff, or BLMs looking for Foe's requiem, or some other synergistic debuffs.