As far as the trap bar, I like them for people with some type of injury if it is the only way they can pull from the floor.....also, if some people with more physique-related goals just don't like the regular deadlift and can't get comfortable with it and this gets them to pull off the floor, they're fine.
As far as training for functional strength, I think the regular barbell deadlift is a better choice. I've never used a trap bar regularly, but the few times I have I could always pull more than my deadlift, and the guys who I have seen go from trap bar to barbell have more of an adjustment. I feel the fact that a barbell doesn't position you perfectly and forces your body and stabalizers to make more natural adjustments adds to functional strength development.
Also, people I have talked with who train for Strongman, a very deadlift-heavy sport, prefer the barbell for the wide-array of functioal carryover they feel it gives to events, as you're lifting real-life objects and you can't always perfectly position yourself in the center and pull smoothly.