Well, in the good ol' days, you paid once, canceled your recurring charge, but still show up - pay once and you are done. I paid once in 2002 for one of my sites, and I'm still in the directory. This still holds true for several of my clients.
Martinibuster is spot on; it's a trust issue.
Traffic from the directory is nil. Every new or redesign I'm involved in, the Y! directory is an automatic decision, but not for the traffic, and certainly not for one inbound link. Trust is the answer.
As for using the toolbar PR to monitor Y! results, it's a bit of an oxymoron, don't you think? That little green bar's value "factors in link structure and many other variables" as G says. So, the real value here again is trust.