"Summon has the ability to turn a good 3 v 3 or 4 v 4 group fight into a succession of 4 v 1 ganks."

Yes, but it also can be used to break up a 3 on 1, if the lone person is both lucky and clever enough.

"However, I do think it is a useful spell/supp."

I do, too.

"So what I'd propose is that the more people from your cabal or group in the area with you, the greater the chance that your summon spell would backfire and send you to the person you are summoning instead. e.g. something like 20% chance of backfire for each ally in the area with you."

Having a conjurer around can interfere with summoning in more than a few significant ways. In addition, summoning already has a high failure rate, and most characters that have summoning ability also have mana concerns.

"This would prevent it being used to gang as much, but keep it useful for simply getting to an opponent for a somewhat fair fight."

Summoning can certainly be one component of a gang. However, by my observation, abilities like grouptell, CB, bash, hide, trip, sleep, bind, knockout, neuro, and blackjack are equally contributory to gang deaths.

Fair fights are up to the players. Anti-ganging code has already been put in place, and I'm not sure there is much more that can be done without sacrificing gameplay. In light of the drawbacks above, taking a few preventative measures can really ruin a summoner's day. Summoning is not as reliable as you would think for most characters.