My friend wanted to test how much damage he did, I'm a Sorcerer, and I summon Clannfear and Twilight Matriarchs, we noticed when he killed my summons, we both got experience for the Fighters Guild. To make sure we saw this correctly, We fought again, and once more, we gained experience for killing the summons.
On the second test, I watched my XP bar, I started off with 852 Experience in the Fighters Guild, upon death of one of my summons, it went up to 853. Upon death of the second one it went to 854.
While miniscule, it still should be addressed, as people who have alot of time, and don't mind just summoning over and over again could probably farm the xp this way, though it will take a very long time.