Okay, so I recently changed my long-time standpoint on this.
If I'm recruiting for a real Purple Squirrel or asking a LOT from potential candidates (for example, I'm currently performing a confidential search for a client in which I cannot divulge the company, the location or the specific job title - and neither do they in the phone screen) I think ultimately it's worth it to make a spliff available if someone helps close a deal by directing me to my superstar.
However, this is not my normal practice. I have been in the business long enough to have an excellent network that provides me referrals all the time for the types of positions I generally recruit for.
I still think in general it's bad for the industry because it creates an expectation. I just watched it bite a friend of mine in the ass. Recently, he called a candidate he had a long standing relationship with. Originally, he blindly called into a company and talked to this individual who directed him to the eventual successful fit, for which he paid $1,500 as a referral fee to the blind find. But later, he developed a req for a position the original blind find was perfect for. He called, pitched the gig and the blind find became his next placement.
So recently, he called the placement back to get some intel on a position he was sourcing for and guess what?
The placement wanted another $1,500.