Law vs Doflamingo was Law being ineffective. It made Doffy look cool but it was just cat and mouse.
Vergo was good for plot reasons not for the fight itself.
Because the Admirals are going to get developed, each one has had their personality explored in their intro arcs. Kidd's been around since Act 2 and hasn't done anything. Preferring a tertiary character taking down an admiral over a crew member is insanity. It'd be a waste