NJ Mom - it's hard to imagine that having the additional full bath was the issue for your neighbors' difficulty selling the house.

We refigured our laundry room and powder room to add a shower to the bathroom when my MIL couldn't make it up and down the stairs when she visited, so she'd sleep in the den, and had a shower. I'm so glad we did it. We don't use a lot, but it comes in very handy sometimes.

I'd say if you have any rooms on the powder room floor (like a den or office) that might be used for a guest room, put a full bathroom in there.