Using the cross platform nature of BOINC is an optional benefit, not a necessity. You can target whatever platforms you want (Windows only, Mac only, Linux only, Nvidia/Windows only...whatever you want). Also, it isn't necessarily difficult to write code that is easy to recompile for another platform. Depends on the nature of what you are doing. I agree though that I'm not sure Gridcoin is providing any motivation for private BOINC projects. They can already be implemented for free after all and I'm not aware of a lot of them out there. Adding the ability to pay for whitelisting with Gridcoin doesn't seem likely to attract more.