I've been doing pure agi at the moment because extra agility adds armour on its own, as well as dodge and parry which this RE focuses on. Parry blocks an entire attack, while armour just reduces the damage of those that get through. Which would make dodge/parry tanks quite spiky damage wise if not for the stagger talent.

In terms of having excess mana. I generally find I have to depend on sanctuary, just to keep it up on trash pulls due to using consecrate and frost shock.

The benefit of the deflection talent is that it happens passively. If i was wielding a 2h weapon and all the rest of my gear was the same, i would have 30% parry which would convert to 10% spell damage reduction when ghostly strike is up (which is not hard to maintain with the ghostly blow RE) Vs an 8% that must be activated and consumes combo points and energy that may be better used on panache to reduce stagger damage. and yes, flurry does not require talent points, but you have to take deflection anyways because 1% parry, stamina, and crit resistance is hard to ignore.

Flurry of blows is situationally useful. I see it as similar to shield wall, or spell reflect. The added melee haste, although a nice flavour addition, and a bit more damage, doesn't really do much aside from its situational spell mitigation bonus.