JamesY wrote: 'That's against the rules' is not the same as 'that's a dick move'. ..... It'll police itself.
I don't see this as a rules thing - without some very tortuous reading, you can summon whatever you like, and bring back units indefinitely.
And that is fine - IF it is taken into account when setting your armies up. If you have one Necromancer and a bunch of skeletons and zombies, endless summoning is not going to be an issue. It is exactly what that army should be doing.
If, on the other hand, you have Nagash and a bunch of Necromancers, deploy less models (a lot less models!) at the start of the game. Hell, have Nagash and his necros be the only thing on the table at the start. The Battleplan you are using may also have a powerful effect on what will work.
What I am saying is, the summoning rules in and of themselves are not broken, they simply... are. It comes down to the players to sort out how they want to approach them, ranging from 'no summoning at all' to 'summon what you like but reflect that in your starting force'.
It is just another component of some armies, one that should be treated in the same way as some of the more powerful units.