The trouble is trying to run a video interrupt with other applications running on the same processor leads to problems with interrupt latency, you cannot gaurantee that you will enter the video interrupt when you expect. Video timing is surprisingly tight which is why its better to run the video timing on a dedicated processor.
If the shift register doesnt get reloaded with video data in time on some lines and does on others then you will get jitter on the image.using a dedicated processor you put it to sleep between video interrupts so you know exactly how long it will take to reload the shift register.