You can’t block all of the CQC abilities via CoA, Doflamingo still prompted to dodge MES for example:
To be fair I think Law haki was stronger then Doffy haki, as example Law could cut though Doffy CoA, but Doffy couldn´t cut though Law CoA.
So it make sense for Doffy to dodge the MES attack, I believe guys with stronger haki should be able to block MES.
Overall CoA only come in game for hax ability, but also for close combat skills of Law it could became useful.
Even though having the worse haki, Doffy was the best fighter to deal with Law, speed,power and his df make him it easier to target and beat Law.