One advantage that comes to my mind is, if you use Poco classes for Orm mapping, you can easily switch from one ORM to another, if both support Poco.
Having an ORM with no Poco support, e.g. mappings are done with attributes like the DataObjects.Net Orm, is not an issue for me, as also with Poco-supported Orms and theirs generated proxy entities, you have to be aware that entities are actually DAO objects bound to some context/session, e.g. serializing is a problem, etc..