Am I alone in finding the thought of unexpectedly walking in on someone in a public or work restroom in their skivvies to be...weird? I'm not particularly prudish, but I don't need to know what colour panties my boss wore to work.
I'm not a modest person - stripping off for appropriate reasons and in appropriate settings is fine by me. But I agree with you, I do not want to see my coworkers in their underwear and would consider it very inappropriate for them to change in the public bathroom area. It's an office restroom, not a restroom at the beach or gym.
I would expect a stall to be used for that purpose. If the restroom is so busy that all stalls are occupied then the person changing would be equally underfoot changing outside a stall as in one - I have never worked in an office with enough space for somebody to stand in the middle changing without blocking the sinks or doors!
I do agree that using a toilet stall as a changing room would be inappropriate if the toilet is busy, they should wait or go elsewhere.