There are two thoughts you should consider in your theorycraft:
- In a ten minute fight, you can use Selene ten times as there is no RNG component to it. With your math that means it'd be a total of 4.5s reduced to the group (I haven't fact checked this, but it looks right at a glance) [EDIT] Actually, you should probably consider normalizing the units from GCD to S or S to GCD and be consistent between it all because 0.45 GCD reduction =/= 0.45s reduction - its not important in the way you've done your math but it will be important once you start adding in things like Greased Lightning, Huton, and Ley Lines, as an FYI
- While the BLM may arguable get the best increase in Arrow (with resource management in mind), Selene is still providing a boost to the entire raid. It's difficult to quantify your actual damage increase is dependent on the skill of your players, the party composition, and other factors such as spell/skill speed as that reduces the effectiveness of Haste. Your BLM has to be doing more damage with their Arrow buff versus the entire party getting 4.5s reduced (based on point #1). Take into account that if the BLM is 18% of your raid damage, that reduction in time is only 18% of the total raid reduction so its effectively 0.67s reduction for the entire raid GCD (probably worth more considering how BLM works in this scenario).
WHM has the following that is better:
- AoE Healing (Divine Seal affects all cures, only the healing potency boost of Synastry affects AST AoE heals)
- MP Management (Free Medica from Assize + Freecure say hi)
- Enmity Management
AST has the following that is better:
- Burst Single Target Heals
- Party Support Buffs
Both have similar kits when comparing WHM vs Diurnal AST, just a matter of which kit may be more suitable for the fight / playstyle of the player.
@Topic
AST doesn't have any glaringly terrible weaknesses. Noct can use some polish to be more suitable and unique, but otherwise AST is in a good spot. It's what I feel a more technical job versus WHM and SCH as you need to not only be aware of your own toolkit but the nuances of everyone else's kit to maximize the potential of your card usage.