Sorry, but that moment was absolutely stupid.
The most cowardly befoon challenged Luffy, fully knowing that he has no way of beating him.
Usopp might as well not exist and it will not hurt the story in the slightest.
I disagree.
I loved that fight. Usopp did extremely well considering the difference in power.
I also think he will be crucial element in fight against Imu, victory might be near impossible without him.
All speculation for now, of course but without Usopp, Perona would have soloed the crew in Thriller Bark.