I cringed when I first read this, but frankly some of the suggestions are even worse. Yes they will degrade, but if you are dead set on displaying them there here are ways to protect them. There are five key factors to preservation of a display, (1) light, (2) temperature, (3) pollution, (4) moisture, (5) time.

Light: UV light is the enemy. Unfortuantely the sun is not the only source of harmful UV light. All light sources have some aspect in the ultraviolet range. So direct light exposure for any time should be avoided. The case should be in a controlled lighting environment. Any room where the light is turned on/off frequently should be avoided. Instead a consistently low lit room is advisable without windows facing south. UV films and conservation glass on the display case would be helpful in avoiding exposure. Direct contact with glass, which of course is reflective is very bad.

Temperature: Before placing cards in case, place cabinet where you wish it to be and place a high/low thermometer in the cabinet to monitor temperatures at various times of the day/year. Consistent temperatures should range from 45 - 70 degrees (with 50 degrees being ideal). Under glass is a suffocating, stifling hot environment.

Pollution: Where to start? Case should not be open and shut variety (or see a lot of use), but rather remain closed. Under glass the constant lifting every time you got a new card, some 524 times would be very bad. Plus, if the contents of a cup are spilled on the desk it will run under the glass. To avoid Off-gassing which can degrade cards the display case should be made of metal, plexiglass, or sealed woods. Using a silica gel pack in the case acts as an internal buffer and pollutant absorber. These act like stay fresh packs in beef jerky.

Moisture: The ideal relative humidity for archival display is 35-50%. Too dry and the card will warp, or crack. In an area of high humidity such as a coastal area, or the southeastern U.S. a dehumidifier is almost a must, and vice versa, a humidifier will be needed for drier regions.

Time: It is not recommended that any art, especially thin cardboard stock be displayed for longer than three months a year to maintain quality.

Of course, living in a sealed bubble with your cards is also not an option. I hope this helps.