We've taken this optional - not once, but twice - as we took the Best of Eastern Europe trip two times, ten years apart. Both times we felt it was worth it and we saw somewhat different areas and rooms the second time. I too am seriously claustrophobic, but it's hard to imagine anyone would suffer on this tour. The rooms are similar in size to what you might see in a hotel - of course the hallways are perhaps less roomy, but the salt mines are hard to imagine as real unless you see them.
Additionally it's nice to have a trip a few miles outside Krakow and see the countryside. Plus, both times we had a really nice meal following the tour before returning to the hotel.