I am sometimes using the "invisible hills" to simulate highest points of flat hills and so on - you may try and have a look on my map of military training area Libava (not here, though, you'll have to go to the SPMBT Yahoo group). On several places this is used, as well as on my Kladno map (same file location).
Or it can be useful when you're creating a map of very flat area (for example banks of a large river with very slow rise of altitude) - you can simulate for example levees etc.
In fact as I prefer to use scale "10 SP height units=20 meters", I am using this in such cases quite often as 20m levee is really a bit too much
)