Brand loyalty is stupid because you miss out on some wonderful discs from other manufacturers. Every golfer who has hit a tree or carded an ace will tell you that this is a sport of inches. Small differences in discs add or subtract those inches in flight that will definitely make a difference in your score. No manufacturer has the perfect line up.
Conversely, plastic loyalty is a perfectly legitimate obsession to have. Throwing mostly one plastic is smart. Grip is one of the most key aspects of this sport. Being consistent eliminates one variable in your throw. If it so happens that the plastic you prefer is only offered by one manufacturer, then so be it.