I have thought about the contracts between them and what possible benefits the spirits gain from working with humans and i sadly can't really think of any.
From the start, simply coming to earthland is painful to them. The longer they stay the more it hurts and slowly kills them. That only happens if they come to Earthland with their own magical power. If they are summoned, it's not hurting them.
They take on a servant role to their human masters. Not quite, they benefit as well. They're (apparently) not a sort of magic that was created by humans, but rather discovered (this is my theory). We know that Spirits and Demons from the book of Zeref have a history together, so it might be that they were either created at the same time, or appeared after the demons from the book of Zeref manifested into the human world.
The time difference of a day to 3 months between the worlds means they hardly get a break. From what I understood, that's hardly a problem, and it could be that they recover much faster in the spirit world than in the human world.
One day is around 15 minutes. So even if Lucy were to not call a spirit for a week. They are getting less than 2 hours of peace before being called out again.
No wonder Aquarius got so mad when summoned. She never had alone time.
The rules and conditions seem so odd too. On one hand it seems like spirits can come and go as they please. Even when not under contract. The spirit king for example summoned himself no problem. Yet so much was lost just for Lucy to summon him once. He appeared the first time because it was one of his spirits calling, and because one of his spirits had "commited" sin and was in need of punishment. He can't probably go and come at his own will, since he's probably one of the strongest entities of all existence.
Yet the rules seem scewed. Like other spirits fearing there keys falling into the hands of Karen because of her abusive actions. Yet that suggests they can't decide the rules to the contract or even decide to not form one. We saw Loke casually null his contract with Lucy to go play on Tenrou island without even asking her. To be honest I never understood the rules of contracts and why you need them in the first place, but then again, it might be as not to abuse the spirits and for them not to abuse their stay in the human world.
Humans seem to suffer no consequences to abusing and misusing spirits yet spirits can be punished with banishment and death if the summoner is killed. Even inadvetantly. With this I agree.
Spirits go from a beautiful immortal world and life doing whatever they want to a world of pain and suffering to act under the whim of any human that finds their key and is bound by a contract like a slave that has them responsible for their human's own stupidity.
I just don't see the up side.
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