A Boar animal companion receives Alertness for free at 1st level. wizards and sorcerers get this feat temporarily while their familiar is summoned.
The Spirit Shaman base class receives this feat for free at 1st level, as part of the shaman's Spirit Guide ability. Add the fact that Spirit Shamans have Diplomacy, Lore, and Survival as class skills, and the Spirit Shaman is arguably the base class best suited for the Harper Agent prestige class.
If having a familiar, unsummoning it will make a character lose Alertness permanently. This is especially annoying for characters that want to enter into Harper Agent prestige class. Tested with 1.05 patch.
This feat will allow certain PrCs to continue spell progression in both arcane and divine classes.