Yes, you can use the Dish coax for an antenna, but be aware that there might be special Dish switches hidden in-line somewhere, and those would most likely have to be removed (and maybe replaced with ones (powered 3-way splitter, for 3 tv's) that work for OTA systems) if going to just an OTA antenna you install on your roof. Those Dish switches operate at frequencies outside of OTA tv broadcasts, and will cause problems if left inline.
OTA = "Over The Air" IE: broadcast tv.