I certainly have; that is not unusual. These attorneys are trying to win any way they think they can.
In most cases, once a file gets to a JDB, the best evidence--a contract between the debtor and the OC--is nowhere to be seen. Although civil procedure usually gives a way to "fix" this by establishing the existence of a contract without the document (the "lost or destroyed instument"), that can be difficult to do and get the evidence accepted by the court.
Instead, a cheaper way is to claim that the debtor, by not paying the debt, is holding onto property that rightfully belongs to the creditor.
The definition of "unjust enrichment":http://legal-dictionary.thefreedictionary.com/Unjust+Enrichment
[U ]njust enrichment is not based on an express contract....[L]itigants normally resort to the remedy of unjust enrichment when they have no written or verbal contract to support their claim for relief. In such instances litigants ask a court to find a contractual relationship that is implied in law, a fictitious relationship...1
By raising this CoA, the plaintiff is asking the court to establish a contractural relationship in the absence of the contract the plaintiff does not have and/or cannot obtain. This frees them from the more bothersome (and potentially expensive) processes of trying to get the contract from the OC and/or establishing a "lost or destroyed" instrument.
Or, in plain English, the Plaintiff is saying "The defendant is holding onto stuff that is rightfully ours and we want the court to tell the defendant to "hand it over"."
Unjust enrichment has three elements. First, the plaintiff must have provided the defendant with something of value while expecting compensation in return. Second, the defendant must have acknowledged, accepted, and benefited from whatever the plaintiff provided. Third, the plaintiff must show that it would be inequitable or Unconscionable for the defendant to enjoy the benefit of the plaintiff's actions without paying for it...
The doctrine of unjust enrichment also governs many situations where the litigants have no contractual relationship. ...1
In the case of an OC? It would not be all that hard to establish unjust enrichment since the OC did provide something of value for compensation which was of benefit to the debtor. However, in the case of the OC, it's unlikely they'd have to resort to unjust enrichment because they would have sufficient documentation to establish the contractural relationship to the satisfaction of the court. This would be true even if the contract itself were no longer available.
In the case of a third-party debt collector? Well, if unjust enrichment is alleged, it should be clear that they have no evidence worth squat since they have to resort to a quasi-contractural remedy that probably does not apply to them, even allowing for the exception for non-parties to a contract:
1.) A third-party debt collector does NOT provide something of value to the debtor-defendant while expecting compensation in return (it's their client/the OC they bought the paper from that provided the service);
2.) The debtor-defendant did not acknowledge to the debt collector that they owed the CA anything (they don't); a CA is an intermediary for the creditor (hired to collect only) and/or is a likely-distant third-party who never risked anything in exchange for future payment (the debt buyer/JDB);
3.) Since the collector did not render any services for compensation to the debtor-defendant, the debtor is not evading payment of anything (they owe nothing to the collector).
Item #2 is another reason why nobody should acknowledge that they owe a debt to a debt collector: Even if state law does not count a simple "yes, I owe it" as reaffirmation, it is likely sufficient that such an admission would make the debtor subject to charges of unjust enrichment.
But, is unjust enrichment a "slam dunk"? Not exactly:
A court will closely examine the facts of each case before awarding this remedy and will deny claims for unjust enrichment that frustrate public policy or violate the law.1
This is the other motivation:
The third-party debt collector, by raising unjust enrichment as a CoA, is trying to do both of the above to try to collect on a claim that they did not have a hand in originating (they were not a party to the original agreement) and cannot often even prove happened in the first place due to the lack of documentation.
-------------------------1"Unjust Enrichment--legal definition of unjust enrichment" from "thefreedictionary.com. Accessed from http://legal-dictionary.thefreedictionary.com/Unjust+Enrichment