I would say that depending on the location and amount under water and the financial position of the owner, this could possibly be the worst advice I have ever heard (but to each their own).
For other locations (say Palmdale or maybe Phoenix AZ, possibly Denver Co) anywhere there is almost unlimited flat build-able land it may even make a little sense.
For instance I know of a Guy in L.A. who is a 100K under water on his home (he still can easily afford it currently), he ask if it would be a good Idea to walk away.
I said are you planning to live the north L.A. area for the next 10 years ???
He said yes.
I said why ruin your credit (possibly your employability) when in ten years it will all be water under the bridge most likely.