In principle, the idea of evil Espers that had to be subdued and enslaved was an intriguing one. It certainly dovetails nicely with the Lucavi of FFT; now we know what the other possesors of the Zodiac Stones would have become had they been corrupted.

It felt like the Esper designers took Anima to the logical extreme. The concepts of slavery and bondage (especially with Mateus and Zalera), along with large monsters like dragons and tortoises echoes rather explicitly the enslavement of Rabanastre and its people. So they fit in (although in a disturbing way).

As for actual usage, they are generally overpowered for most regular monsters, so I use them the most for Marks and some bosses, especially ones with elemental weaknesses to exploit. But unlike some of the most devatating summons (like KoTR), I don't generally use them against the toughest bosses and Elite Marks; there's simply better ways to deal damage.

I think Anno Domini means 'I it.'