How would I apply proficient vs non-proficient for a climb check? Easy, the proficient climber has the opportunity to assay the climb and choose an easier route (if one exists). The non-proficient climber does not. So the proficient can find an easier DC, and they can even take some time to share the details of that route with the non-proficient. The actual check might be the same DC, but the proficient has a chance of changing that DC. And, depending on the resultant DC, I might choose to not even require a roll from the proficient.
Which brings me to a question: for those arguing that it must be the same DC for everyone with the same results on a strictly pass/fail basis, would you ever allow a proficient person to auto-succeed while requiring the non-proficient to roll? If so, how is that different from setting different DCs, or varying the results of success?