It just starts to lose a bit of its potentcy after being mixed awhile. And the longer it is mixed the more likely chance of infections. BAC water only stays good for so long once seal is broken. The refriguration will make it last longer, not break down as fast, and slow bacterial growth.
I have a buddy that used 1 year expired Test and still had a good cycle, just lost a lil potency. Ddint get the sides as bad as he normally did, and his where like clock work.