String Death & A Little History on Poly Strings...

As far as I know, there has been a perpetual debate on whether nylon strings (like multifilaments and synthetic gut) go "dead".

Even among those who believe that nylon strings can "die", they carry very different notions of what happens. 

Some claim a sudden loss of tension and hitting the balls out. Others feel their stringbed going flat and playing completely lifeless with little to no elasticity.

There are also some who believe nylon strings offer only about 10-15 hours of comfortable play, or storing the strung rackets in the trunk of a car parked in the sun hastens the strings' death.

So far, the best information I came across online was from a forum post. I think it explains string death extremely well.

For ease of reading, I have reproduced the post (link) below with permission from "TennezSport":
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It's all about elasticity vs. resiliency in string that makes them perform the way they do. NG string has the highest level of both of these elements and thats why this string is by far the best string on the planet. Mathematically elasticity is the ability for a string to stretch and resiliency is the ability of a string to recover to it's original state. 

The issue arises as no string is 100% resilient and cannot recover to 100% original state, so we see tension loss. With NG, multis and SG string the elasticity is high and the string will lose tension as resiliency is lost as we continue to play. This will continue until the string breaks or all resiliency is eventually lost and the string goes dead (for non string breakers). NGs resiliency is so high that it almost never loses it and will perform great until it breaks, after it settles in.

Poly and co-poly strings have very low elasticity and low resiliency, so they are stiff and lose tension quicker than they counterparts. Plus, poly/co-poly strings have a max stress point (varies by string makeup) where you can pull it so tight that the string plays dead from the beginning; it's surpassed it's resilient point and cannot recover at all; resiliency = 0 (kind of like when you bend a plastic card and suddenly see a whitish line at the bend; the line shows the plastic has exceeded it's recoverable state). Because you have a high stiffness index with low elasticity and resiliency in poly/co-poly string, stringing lower is crucial for the life and best performance from the string.

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Under the same topic, the forumer also mentioned about how and why poly strings came about. I think it is important to know.

Likewise, I have reproduced that post (link) below:
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Thanks SteveI and Timball for your kind response. However I cannot take all of the credit as the team here has spent a lot of time with some great string engineers to understand this mess. We recently spent a great week with Jay Cee where he dazzled us with his knowledge of poly/copoly string design; our heads were spinning.

Timball, you are asking very good questions and I will try to answer as best I can. JC told us that when they originally developed poly string it was for pro players only, durability was to be for one match or 6hrs max. Because of the higher stiffness index you could string lower and not lose control and it was harder to break, especially good for clay court players. The opinion at the time was for 5-10% lower tension than you would use for a NG or Multi/SG string. Tension loss was not a consideration because pros would restring almost daily. 

However, when rec player heard about this "Magic String" that their fav players were using and they heard the word "Durable" they as you say "jump on it". Most poly manufacturers were not prepared for this and just sold the standard poly string as requested. The second problem was that most rec players did not adhere to the 5-10% reduction and strung at normal tension; elbow problems rose immediately. Several companies like Kirschbaum began to address this issue with the concept of copoly strings, trying to soften poly string and give them a longer playable life span for rec players. At one point Kirsch suggested not to string over 57lbs (25.9kgs). All companies are still searching for that soft, spinny, durable magic string today.

Finally to answer your last question, poly strings are better today and good for some rec players who understand that poly string has little more life than NG/SG/multi string in the sense of playabilty. You may not break the poly string but it does lose it's playable life faster than NG/SG/multi string. Poly string is good for you if it fits your game and your level of ability, as long as you understand how to properly use it and not hurt yourself. Hope this answers your question.

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JC, or Jay Cee is a forum name used by John Elliot. He is also known as the "Father of Polyester Strings". You can take it from him that he KNOWS what he is talking about.

I hope this clears the air for those in doubt. 

Ultimately, re-stringing regularly and playing with fresh strings is to protect ourselves from unnecessary injuries. Why save that tens of dollars on re-stringing and risk spending thousands treating wrist/elbow pain later? 

Enjoy your tennis safely!