I think this really depends on the party setup.I'm assuming the reason people don't open up this way for the most part is that if say there's a dragoon in the party it's worth staying in deliverance on opening for that higher fell cleave crit chance.Given the long cooldowns on some of the party buffs in general you're probably still losing more dps overall.
Most competant MT's can stay in their offensive stance when dealing with a single target boss anyway.
Hell you could just open with a tripple cleave on warrior and hold agro on boss that way and let pld switch after anyway.