Guildhests are also a way to help guide new players into party play by teaching common gameplay techniques like pulling an enemy without alerting others nearby and learning to take down healers first. Yes it's elementary stuff but sometimes basic is best.

But yes, as Bixby said you have the initial bonuses for the first time you complete it but after that there's no real benefit to doing them other than for the Challenge Log. However, much later on in the storyline you are actually required to undertake a number of guildhests to progress in the main story, so it also helps to at least have them unlocked so you can quickly get them out the way for the main scenario. But it's up to you.