While OSSC has a lot of features that full blown video processors have, it is designed first and foremost as a lag-free line doubler. It’s designed to integrate into any setup without adding any additional lag. Full blown video processors are different, they offer functionality like pan/zoom, sophisticated deinterlacing and advanced picture processing that’s just not possible without a full-frame buffer.
If you need that functionality, adding an OSSC to many video processors gives you excellent 240p picture quality (many video processors don’t handle 240p well at all) and all the advantages of a full video processor, as well as some disadvantages (more lag, longer 240p->480i transition times)
Of course, not everyone does need that kind of functionality, as long as the scaler in your display handles the OSSCs signal well (either in line double or triple mode) then there’s no need to add a video processor too.