If you're a competitive powerlifter, the trap bar can be used as an accessory exercise but not necessarily as a main movement. For most people that compete in powerlifting, the need to keep the basic lifts in the program is essential. Granted not everyone needs this but if you compete in the bench press, squat and deadlift you should probably practice these lifts, in some form, the majority of your training.
But if you're not a competitive powerlifter and need a good change of pace from pulling with a straight bar, the trap bar deadlift is a great option. This includes athletes and others who use strength training as G.P.P.
I liken this movement to a non-competitive lifter going between the hang clean and full clean (or power clean). While not the same thing, it offers a great change of pace, yet still maintains the integrity of the movement.
The trap bar is also a great way to increase quad strength, and it takes a bit of stress off the lower back as the handles keep the center of gravity closely aligned with the hips. For strength coaches that battle with sport coaches about the safety of the deadlift in their programs, the trap bar is a great compromise.
Let's face it, chasing the Big Three (squat, bench press, deadlift) can get tiresome, and having an acceptable substitution that can be used for several months might be just what you need to keep the competitive fires burning.
Finally, the trap bar allows you to pick something heavy off the ground and there's nothing more awesome than that. If you trap bar deadlift 900lbs, no one is going to accuse you of being weak. The only caveat I will mention is the amount of normal-height lifters using the high handles on the trap bar in order to satisfy the ego; just stop it.
Don't be so stubborn in your vision to leave this lift out of your training because it isn't a competitive lift – expand your vision a bit without sacrificing your principles.