An astrological Sign typically can be considered an environment or situation whereas Planets represents a more tangible energy (like a Human being) which operates in that environment. In every situation or environment there are certain qualities or energies which are more useful and certain others are not.
For example to stitch a cloth you need a needle and to fight a battle you need a sword. So needle is debilitated in battleground but it is exalted when it comes to sewing the cloth.
Now lets see what is Pisces as an Sign (environment) -
Pisces is the last sign of zodiac and widely known as sign of "moksha" or "emancipation". However I like to call it a sign of infinity. It is the wonderland which exists deep under the sea with beautiful and shiny fishes floating around in soft and calm water; effortlessly travelling through the beautiful and endless waters. It is like you are seeing a fancy dream in sleep, much like Alice in wonderland where every imagination is a possibility.
What is Mercury - Mercury is our mental, analytical and calculative power. Mercury calculates, evaluates, find a best way to do thing or find solution of a mathematical problem or calculates how much money to spend to keep the budget in check. It is THE most discerning and discriminating planet.
But wait... If I am living in an environment of Infinity, why would I care to calculate, analyze or discriminate? Calculations, analysis, discrimination and doing math is only needed when there is limitation or restriction. If you have infinite resources available you wouldn't need this faculty.
However…. Don’t think that debilitated planet is all bad. Depending upon the horoscope a debilitated Mercury can impart many desirable qualities and goodies in life. And this can happen even without the usual cancellation of debility or “Neech Bhang”.