Oh, I get all that. I really do, but -
There Is legitimate greyness around the timing of his particular rule and the sequence of rolling attack and defense dice during an Attack action.
And.
In a tournament, I expect my opponents to play to the letter of the rules. If I can't show them, by the rules, that they are taking a currently (not how a future FAQ might rule, but the rules as of the event) illegal action, it would be poor sportsmanship on my part to expect them to not do a given thing simply because I don't think they should.
Everything - except the actual rules - screams to me that Gurzag cannot wait to see his opponent's dice, but I have no rules to support this.
Keep in mind, this is GW's attempt at a tight, competitive, tournament game. Players should expect a different type of interaction in this than they would in, say, a narrative AoS game.