someone has stated that a crack is illegal which I find surprising but I'm not that au fait with present US laws. I have seen many windshields with a crack across one of the 4 corners which if illegal would have to be replaced which also seems very harsh in my book.
Modern windshields are laminated, which means they are two sheets of glass with a plastic sheet in between. This prevents shattering like the other windows in the car.
You would have to go back many years to find a car with a toughened glass screen that would shatter. I have a few classic cars with these. Cheaper to replace but not as safe.
Structurally there is no problem as any distortion of the A pillars will cause the windshield to crack. They are designed to pop outwards in the event of an accident.
Normally only a crack in the part of the windshield the driver looks through would result in a fine or a fail at the annual national car test or inspection.
Any crack in any material, glass, steel, aluminium, fibre glass etc will propagate, in other words spread. You are right that temperature fluctuations will also have a bad effect on cracks. Any crack can be stopped from spreading by drilling holes at either end of the crack or as with the modern insurance company allowed ways, a hole is drilled and a silicon like solution injected to seal against moisture.
More expensive windshields have as well as the plastic lamination a very fine electric circuit, much like the one on rear windows for defrosting. These screens can be mad expensive so I personally would check for sure your state laws regarding cracks in windshields and which can be repaired and which have to be replaced.
The insurance repairs are usually free and as someone stated do not affect your earned discount or bonuses.
Stone chips are the usual cause for cracks. They break the first layer of glass to the plastic laminate and if left unchecked they produce cracks but they also turn the plastic laminate white. This is usually seen towards the bottom of the windshield under normal wear.
Always inspect your windshield for both cracks and stone chips. If you see any check with the auto glass firms as to the best remedy. They will give you advice for free and if not go elsewhere.
Hope this is of some help,