"Someone (unfortunately I forget who) mentioned that the best thing an academic can have is a good enemy. Your friends will, perhaps, read through your papers and make a few comments. But only an enemy will read through an entire book, for free, finding every error and questionable statement."

That's a good point that gets no argument from me. Here on HN, we can all appreciate best those fellow participants who hold us to our facts and ask for proof of our statements. I learn a lot from HN participants like that.

Obligatory link to a Paul Graham essay on a closely related topic:

http://paulgraham.com/trolls.html