The purpose of Aerenal Grace on my build is to get DEX-to-dmg with rapiers, longswords, and longbows; in particular, Aerenal Grace is currently the
only way to get DEX to dmg with longbows. [Why Turbine didn't add a DEX-to-ranged-dmg enhancement to AA or DWS when they redid the PrEs is beyond me, but so it goes.] Note that if you want DEX to-hit with rapiers as well, you need to take Finesse feat; but with Precision & bard buffs, I wouldn't worry about it.
EDIT: when you get to lvl 20, you may prefer to use a pair of
Oathblades rather than rapiers, since it has the same crit specs as a rapier or scimitar while benefiting from Aerenal Grace. Plus it's keen, so you don't even need to swap out IC:Pierce.
And yes, Tempest lvl 3 & 6 cores already give you DEX to-hit & dmg with all light weapons plus scimitars, so you can use those instead; although there's not really an incentive to do so, considering Aerenal enhs provide +4 to-hit & dmg to rapiers.
The problem is there aren't any good named shortbows, whereas there are a ton of named longbows (Silver bow, Bow of Sinew, Pinion, etc.). Since Aerenal wins by default on a DEX-based archer.