More info about Fairies:

Selene seems to prioritize silencing casters and giving AoE +Cast speed over cures, so if you use her on SCH to main heal, you sometimes have to pick up a bit of slack.

All summon pets take -90% damage from AoE, as in WoW. However, they still get the poison damage, silence, stuns, and such from the AoEs. You have to be wary of those and once again pick up the slack if your fairy gets debilitated.

You can "place" manually your fairy to spots that are safe for them to heal from, and bosses won't target them with spells. Their auto-heal range is shorter than yours, so you have to do that at times to make sure they heal the tank instead of standing still. Also need to be wary of LoS objects like pillars blocking them from healing the tank.

They do not seem to wake up mobs or cause agro if you "place" them near a boss before the fight begins.

They have endless mana, and have their own threat.