Somehow or another the creature has to keep feathers clean. Try to imagine a T-Rex preening feathers with that jaw. Even with some extra "beak" there's too much that's unreachable. The forelimbs aren't much use either. What's left?
Rolling in the dust might work if the tail was flexible enough to right himself afterwards, and it would probably have to be (everybody falls down sometime).
The forelimbs might work OK for preening another T-Rex's feathers.
If the T-Rex had significant feather-ation, I suspect he'd have to roll or else groom in groups.
No need to thank me for the mental images...
Yes, I know that T-Rex would be too heavy to roll on feathers, exactly--thrash around would be more like it. And unless you're a pack hunter that herds prey into a trap, flamboyant colors on a predator seem like a recipe for fasting.