From time to time students come to me expressing an interest in using plastic mouthpieces. Whether it is to save money or for comfort, students are attracted to these colorful lightweight options. My response is that for everyday playing, nothing beats a high quality, appropriately sized brass mouthpiece. The quality of sound, consistency, and the fact you will be using it day in and day out far outweigh the relatively minor upfront cost.
That being said, however, it is helpful to have a plastic mouthpiece on hand for one particular circumstance. If you are going to be playing in the cold, and especially below 32 degrees, then you can pull out that translucent, hot pink acme brand mouth piece and have at. Your lips will thank you.
So in the end, they are not a good choice to save money, but plastic mouth pieces can save your lips when marching in the frigid winter air.