I'm going to go out on a limb here and say they are pretty useless, in fact I would say they are even a nuisance. The idea was novel when it first came out and it was for a brief moment useful, but those days are over.
I never enable pingbacks because they seem to actually discourage discussion, and are highly abused by SEO spam.
Trackbacks might be interesting from an analytic perpective and slightly useful for the users but I still turn them off, because they also discourage discussion and attract spam.
Ask yourself how many times have you clicked a trackback, have you ever read a pingback??
As toscho has pointed out, sold/outdated sites can start to return spam pingbacks/trackbacks that were at one point valid, in fact I bet there is a market for this.
The pingback system is automated so your loss of control over it has attracted spammers because it's easy to abuse.
The web has evolved away from this type of service because it prefers actual people ( And the search engines try to reflect that as well).