Jim Brennan
VHS Member Old Skool
The only time you'll need to source in SD is if you need realtime output to somewhere that only takes SD and don't have a good, real-time converter/scaler to stick in the signal path.
I don't think anyone is implying a need to source in SD. Just that there is not necessarily a need to source in HD. I do 90% of my paid work in SD from start to finish. My clients are happy. For some projects shooting in HD and downrezzing for delivery is worthwhile, but a lot of times it isn't. People pay more for something that is "better", but actually adds nothing to their specific purpose.