This summonning issue is, as far as I see, one of the big problems in the CR systems. As an example, once my party faced a Dracolyte (a full caster 15th level cleric) and the guy pulled off a damn Adult Red Dragon out of his *ss, for 10 freakin' rounds. Overkill. One can say 'well, it's goes off after ten rounds', but dude, it's ten rounds facing one 15th level Cleric and one CR 15 Red dragon. They WON'T need 10 rounds to tpk even a 15th level party.