I assume the purpose was to show that Kizaru is a light man that Luffy struggled to keep up with, all while other things were happening. He did achieve a few things:
- He removed Sentoumaru from play
- He destroyed Vegaforce 1
- He sent Bonney down below which forced VP and others to head down too, along with the pacifista change.
- This forced Saturn to arrive to be the final command on the Pacifista.
It played its role, and gave us a skirmish, but thats all Luffy vs. Kizaru was until that one hit.
Hopefully they are both down and we get a real fight after all the Saturn business is done. Oda spent like half a chapter the last 4 chapters on Luffy, so putting them both down gives him space to focus on other characters.