Loose isn't a problem and i like poly very loose while others don't. You're just going to get a softer feel, musher feel, trampoline feel, or whatever you want to call it. Stretching the string as it contacts the ball relieves a some of the stress on the frame and you.
Just like the old string and tin can telephone. The tighter the string the better vibrations are transferred to the can / racket.
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