It becomes brittle and cracks.

Cleaned out a vacant unit today.At the back wall next to the slider doors on the inside is dry and brittle in the pics.Don't see any insects there.Don't know what this is.Any Guesses?

It's just too brittle to do it right and this way looks fine from 5ft away.

There is also the issue of it being more difficult to install as the shingle is more brittle, and its much easier for the nails to blow through the shingle during install.Temperature isn't the end-all if your roof is completely shot - shingles can be held in a heated tent until they are ready to be installed, handled more carefully, and hand-sealed.

You will be surprised how a broken pipe will hold its form and still work and could be ok for long time but if that spot is cracked it probably means the whole system is brittle and once dug up will just crumble.

(Bonus if you carry some plumbers tape or a few rubber washers in case some have broken or become old and brittle!)

Pipe was brittle and crushed in places. 

As to the why of what you're dealing with - a lot of insurance companies are doing this because roofs over 15 years of age in Florida are highly susceptible to wind damage because of the brittleness in asphalt shingles that's caused by long term sun exposure.   

Durock seems to be more brittle etc..

The snap bead that surrounds and helps seal the perimeter of the window is brittle, faded and unsightly.