Provoke is necessary due to their being several encounters that require two tanks to trade aggro. Its possible to do this without provoke but EXTREMELY difficult and would require incredibly synergy between you and the other tank as well as less than optimal threat gen creating another hazard for your dps.

Why don't warrior have provoke? I've asked myself that question many times. It feels like an oversight by SE to me. I don't think a class should have to cross class a skill to perform its basic role. I can't think of another class that HAS to bring a cross class skill to function. Many are nice (swift cast, etc) but WAR might be the only that is basically required to bring one.