Summoned units last 2 turns rather than 1
Summoned units get the Combat 1 promotion
The Summoner is a Spellcaster skilled in the summoning of extraplanar beings. The various contracts, and relations built by the summoner allows the nation to summon more skilled assistence from beyond, and for such assistence to last longer then usual.
Summoner is a powerful trait for civilizations like the Sheaim that greatly depend upon the power of summoned units for the backbone of their military. By having their summoned units last double the duration of others' summoned units, an adept player can wait an extra turn to almost double the number of summons (and net attacking power) of a stack of units.