One of the advantages of LEDs are their long lifetime and long lasting constant performance. Ideally, the optics used together with the LEDs should be as long lasting. When high flux requirements meet the need for the color and intensity homogeneity often light guides are used. Typical RGBW-LED light sources show up to 300W power, leading to a heating of the optical material the light guide is made of. When also taking ambient temperatures into account, it becomes obvious, that optical polymers are not suitable any more.
For an example, a 14W LED was used to calculate temperature values. Already at 62 °C ambient temperature the maximum permanent operating temperature of PMMA is reached. For PC ambient conditions of 79 °C will exceed the material's limits. Silicones have a higher operation temperature than usual plastics, but show many other disadvantages when heated up. Minor irregularities within the material (local density fluctuations, etc.) or surface deviations will lead to even smaller usable temperatures. The result is the collapse of the optics and potential additional damage to the system.
For glass those conditions are far below its limits.