I equipped some Touch of Serenity (Enmity -7) Materia on my belt. It's not like -Enmity Materia will suddenly allow you to drop Curaga bombs all day without getting smacked around by the mob, but our only other real options
for this slot are +HP, +MP (35 max), and +Resists, so Touch of Serenity is the clear winner imo...
Touch of Serenity maxes out at -8 Enmity, which doesn't sound like a lot, but you have to keep in mind that you can equip Out of Sight (from Archer) for an additional -10 for a total of -17, which should start to make a pretty big difference if your tanks are adding +Enmity Materia (which many will).