I haven't seen it game breaking either, well only once, but one guy wanted to try out Nagash, which is a whole different ball game when it comes to summoning. And yeah, summoners become HVTs(High Value Targets - I've taken a summoning player by surprise, to his horror, who had bean boasting that nobody could beat a summoning army, and I broke him on turn 1, before he even had a go)
How do you deal with it though, do you play it as though you have to have a unit of type in play in or to enable to the spell, or do you take it a Chaos Wizard, Death Wizard or Slann or whatever always know all the summoning spells that are relevant to their faction?
And then what about summoned units that can summon, like some of the Deamons of Chaos? Do you allow them to also summon in the turn they arrive? In theory this could go on and on, until they run out of stuff to summon, and each time you kill it, they can summon it again. However, given that you have to carry stuff to games, I'm not sure that is likely to be abused, especially as it would also slow the game down quite a bit.