From a Boeing Manual, my highlighting;
(2) Flight adequacy of the windshield shall be determined by visual
inspection of the transparent components (elements) and weather
seals and by an operation check of the electrical anti-icing system.
(3) Removal of a windshield is only required for obvious reasons such
as:
(a) Heat inoperable for reasons internal to the windshield.
(b) Structural glass ply contains cracks, chips, or shatters.
(c) Cracking or shattering of outer glass ply. Refer Dispatch
Deviation Guide for dispatch.
(d) Cracking of vinyl ply if vision is seriously impaired.
(e) Delamination of any plies if vision is unacceptably impaired.
(f) Objectionable pressure leaks.
(g) Vision seriously obscured for any reason.
NOTE: Removal for structural reasons is not expected.
If structural deterioration does occur, the obvious
removal reasons above will have occured well before the
assembly has become structurally unsafe.
point (c) above refers you to the DDG. The Boeings I work on allow for the removal of the outer 'Glass' layer. If it's cracked or 'spider-webbed' one of us engineers gets up there and chisels off the complete layer of glass which restores the visual quality of the windscreen and then you can fly home to replace the windscreen. This glass outer layer is not structural in any way and is utilised as it's very hard and resistant to abrasion from flying at 900kph and from the windscreen wipers dragging crap to and fro across the windscreen. Imagine a plastic windscreen on your car, it would look terrible after not too long. Bare in mind that some windows also utilise glass in the primary structure (although most use plastic/vynyl) but there are generally up to 4-5 layers so a crack in one doesn't affect the strength of the complete assembly.
In addition, below is from the inspection procedure for windows;
(3) Cracks:
(a) A crack is a fissure that has a visible width or depth.
(b) Cracks can start from a scrath or a crazing mark (Fig. 601).
(c) Cracks can be single or dual (Fig. 601).
(d) Cracks in glass will usually occur 90 degrees to the surface of
the pane.
(e) Cracks in stretched acrylic plastic will occur at
a 45 degree angle to the surface of the ply and can become
in - plane cracking.
(f) The cracks in an outer vinyl ply (the ply between the middle
structural ply and the outer ply) are caused by incorrect heat
application.
(g) The cracks in the vinyl usually occur in the window corners and
are within the edges of the release tape.
NOTE: Vinyl cracks are not a problem structurally.
NOTE: Only replace the windshield if your vision is limited.