Firstly, Flash is installed on more machines than any other plug in, web browser, or operating system. Allowing users to join your science project without having to register, download, or install greatly reduces the barrier to entry.
Additionally, the OCG is not meant as a replacement to existing standalone frameworks (like BOINC or World Community Grid) but moreover as an extended computational and advertising platform.
For example, let's say you have a standalone application and also an OCG port. Your OCG application can say a message such as