Kennon Sun Shields keep the cockpit cooler than Cabin Covers. Kennon Sun Shields are more economical because they cost less than an exterior Cabin Cover. That's a very good question. In fact, prior to 1992, manuals produced by Cessna advised against using sun-reflecting screens. But after the Air Force tested and accepted Kennon Sun Shields for use in DOD aircraft, Cessna removed the warning.
Heat is one of the major causes of crazing.
Plexiglass is produced as a flat sheet, then shaped with heat. When it gets hot again, it tries to return to its original flat shape. This stress on the curved areas of the windshield causes distortion of the plexiglass which is commonly known as crazing. Crazing is very often first discovered on the curved portion of the windshield. Heat is produced when the sun's rays are absorbed.