Flagging is acceptable on "disagreement on rewards", so if you don't think someone earns the payout they are getting - it's ok to flag :)

Usually spam and plagiarism are being targetted of the flags, or the typical "I don't like what you are saying" but those are quite rare anyways.

Well at "worst" (or best?) haejin is currently making over $2500 per day with his posts - or more than that, when you take in consideration the SBD value.

In addition to this, haejins upvotes are around 75% self-upvotes AND what you asked about his share of reward pool. Using this tool ( https://steemworld.org/@haejin )to check the amount he is getting from the reward pool, currently it seems to be over 2% of the total reward pool.

Flagging is not only bad, you invalidate someone elses vote BUT this means others will get bigger share of the reward pool. Sometimes you can't only find things to upvote, while leaving the "playfield" for someone to get over 1/50th of the whole reward pool for themselves.