Everybody I've ever spoken to (outside of small JS type devs) has got a linkedin profile and expected me to have one. So many recruiters (I contract now) have used linkedin to contact me based on searches they've done which I have converted into 6 separate projects now (and at £650 per day that's a lot of money).
Depending what circles you move in (I move in big enterprise software and services circles) it's just a given that you will have a linkedin. I have a domain name which directs to my blog and one page profile but even then people still ask for my linkedin.
I don't know what the smaller time software dev world is like but for me linkedin is essential..
Saying you can't believe why anybody in the HN crowd would use linkedin probably shows that you only know about the bubble that you operate in.