This is also an opt-in feature of OKCupid.

I personally opted out, because people's whose profile I viewed but didn't contact are exactly the people I don't want contacting me- which they could do if they saw my profile name/info.

Namely, these are all the people whose profile I looked through and said "meh." If they knew who I was they might contact me when I already determined I wasn't interested.

I imagine it might be the same for LinkedIn; say you were looking to headhunt someone but found their resume lacking. Would you really like them to have the option to then contact you and ask for a job?