I agree to a degree. I give out 50% XP for all Cohort / Summoned / Companion targets defeated.
A L17 Conjurer / Master Abjurist w/ a Maximize Rod can drop 6 Greater Air Elementals in 1 Round. That's just mean to not give anyone trying to combat those some form of XP. If a Conjurer is going for the "overwhelm" card, in 2 rounds they can summon 10 Fiendish Girallons and 10 Huge Air Elementals. Not including the use of Celerity and stuff like that to get extra turns as well.
By giving out 50% the PCs facing it get decent XP, but not enough to warrant hanging around just killing the summoned creatures to gain scads of "easy" xp.