Round-robin scheduling: allow listing a process multiple times?
The advantage of this approach is that round robin scheduling is very efficient, so you don't need to rely on a more complex scheduler, which would steal cycles. Introducing longer time slices for higher priority processes would decrease the responsiveness of other threads and make it harder for the OS to jump in to deal with an interrupt and so forth. Maintaining separate lists of processes would require a much more complex scheduler, which would be more expensive in terms of cycles. I don't see how there would be any problem with fairness, though. I guess the problem is that the granularity is fairly course: processes can only have integer multiples of the basic time-slice.

One negative point is that removing a process would be more costly, as each occurrence of the process would have to be removed from the queue. Perhaps this can be done lazily, though.