The Scarlett and I had the opportunity to ask Tim Gunn why our favorite judges have been missing. Tim explained that Nina was attending some of the European Fashion events for part of season six. Michael was also overseas (in Italy I think) sourcing fabrics for his new collection. The problem was that these trips were planned before the filming schedule for season six was determined.
We know that the challenges are filmed back to back with no time between them, so a five-day trip could easily result in missing three challenges.
I like seeing a guest judge once in a while.