Given that Kensai Monks are forced to take a ranged weapon as one of their Kensai Weapons, we can assume that Kensai Monks are intended to be reasonable ranged attackers -- I believe Deft Strikes exists mostly to support that. Most of the other ways to spend ki at level up until level 11 either *require* (stunning strike, flurry of blows) or are *most useful* (dodge/disengage) in melee situations, which most bow users try to avoid (and monks would be generally better at that with the +15ft movement they'd have at level 6).

Deft Strike allows a ranged Kensai Monk to spend Ki to increase their damage. Seems a welcome option. Allowing melee monks to use it for the few situations where immediate guaranteed extra damage is helpful is just icing on the cake.