* It's where my "general business" social network lives. Which is different from my company network (which lives in a CRM). My twitter social network (public facing me) and my facebook social network (mostly private me) and my app.net network (... erm... mostly tumbleweed ;-)
* The rest of the world uses it. Like it or not a stack of people go there to find out about people. Not being there means those people miss you or trust you less.
* It has some level of value that a private site does not. Recommendations etc. are more than mildly useless - but slightly less useless than a random quote on your own web site.
* The network is useful. I've managed to contact and strike up relationships with folk because of my LinkedIn connections. People have reached out to me from my network. Some of those connections have resulted in non-trivial money for my business.
Sure - having LinkedIn as your only profile on the net is foolish in the extreme. Having it as part of your identity online has been useful — for me anyway.