There are situations where Topaz Carbuncle or Titan-Egi can be a great help to tanks and I would have them summoned for the purpose of assisting the tank

2 cases being

Last boss in the Sastasha dungeon and the first boss in Cutter's Cry

Both of these bosses at about 50% of HP will spawn lots of adds

In both these dungeons I have had the tank turn over the main boss over to me to hold for the remainder of the fight while the tank holds adds

A note to any ACN or SNM reading, this a few tips

1) This only works with Topaz Carbuncle or Titan-Egi and having a Clued up tank

2) At the start of the fight send it pet using SIC command at the same time as the tank DON'T use a spell to send the pet in

3) Keep an eye on your pets hate and heath meters as well as the bosses health bar

4) At the point that the tank turns the boss over to you then STOP DPS'ing on the boss as you will needed to save MP and only focus on healing you pet