In Final Fantasy X the Summoners are all White Mage-class people. White Mages have absurdly low physical attack and defense, and are really only good for casting healing-based spells with the exception of an occasional Holy spell. Hence, the Summoners needed guardians to ensure their safety during the journey.
Kingdom Hearts is completely different, because even a Mage like Donald can hold his own against a reasonable amount of Heartless. Each fighting character (ie: Riku, Sora) in this series can defend themselves, but since they all have weaknesses, they watch each other's backs. Their relationship with one another is more similar to a team (which, really, is what they are -- however fragmented it is at times) as opposed to a person and their bodyguards.