We might have question that if we didn't complete some quest can we just buy the nft from Opensea or Quix.

This have been a question without clear answer for a long long time. But team usually says that it's better to mint them yourself(of course).

This is what i found on Optimism discord chat today (10 Feb 2023):

However, those Quest NFTs are quite cheap. I also bought some earlier too.

More update...

Team has confirmed that Quest NFTs will get you an Optimism PFP!
OP airdrop has very interesting story and I think holding an OP NFT will give you some advantage for sure.

What do you guys think btw? Hope this info help you in decision making.

Cheers,
Nanobro