Without this function built in, I find myself having to use raster math to do unit conversions between meters, feet, and decimal degrees. Then I usually reproject to make sure x, y, and z units are all the same. For large rasters this is a lot of extra time that could easily be avoided with a Z factor parameter built into the graphing engine. Note, I'm almost exclusively using data in GCS_WGS_1984 so this is always an issue for me.
