User blog comment:D.Dragon123/Why Haoshoku haki is useless./@comment-3119203-20130115000608/@comment-3119203-20130115205311

This is what makes me think that there is a limit to how many bursts of haki one can do. I'm pretty sure that Luffy could have knocked out a few more fishmen, but there might be a limit to how much you can do with this power. Hence the need for training.

I always wondered why antagonists hold back at first when fighting protoganists. Just go all out and kill the guy already.