Pamela Jones, of Groklaw, thinks that OSI has a problem in Open Core. I think she has it backwards. The Open Core (most notably SugarCRM) folks have a problem. They'd like to convince people that they can achieve the open source effect by having two versions of their code: an open source version, and a proprietary version. They'd like to convince people that their proprietary code is going to benefit from the open source effect. Good luck with that! The open source effect is an integral part of having code distributed under an OSI-approved license. You can't slice and dice it like you can with property rights, having surface rights, mineral rights, water rights, etc. Not open source? No open source effect for you!
The more they differentiate their proprietary version from the open source version, the more obvious it becomes that the open source effect isn't contributing to the proprietary version. The less they differentiate it, the less reason to buy the proprietary version. This is a narrow plank to walk. If the Open Core folks can figure out how to stay on the tightrope, and make lots of money, hey great! Good for them! They're not going to be a success by trying to sell their proprietary version as if it was open source, though. That dog don't hunt.