Dual licensing and Open-Core are not same things
Dual licensing is when you require copyright assingment on all contributions, and they offer same product under both GPL and proprietary license. It must be same product, bit for bit. This is valuable when you have something like database, so if people link it with proprietary programs, they will want to purchase proprietary license in order to avoid open sourcing stuff. If you use it with other GPL stuff (like LAMP stack), then you don't need proprietary license. MySQL used this model until Marteen Mickos decided to take it open-core route.
Open core looks similar but its not. It also requires all code to be owned, but you have two products. A crippled good-for-nothing GPL version, and hyped up proprietary version which has more features. Open core make money only on proprietary software, selling proprietary version of their product. Open source version is there only for PR, there is no developer community; only the company paid devs work on the project. They reject outside contributions if they compete with their proprietary-only features. Open Core companies operate like any other proprietary company, except they claim to be open source.
It is sad to see open core ruined another open source project. Marteen Micrkos destroyed MySQL and Eucalyptus. Another victim of Open-Core is Compiere who got bought by Consona for pocket change. Open Core model only drives developers away, and leaves project to die.
Ex Microsoft employees have started some open core companies to compete with open source.