The argument for machines possibly being sentient lies in the comparison: Humans are biological machines, and yet we are sentient, why can't another type of machines be? For that qualification to be met their programming must have the necessary complexity and the ability to dynamically adjust itself to novel situations. There's a better argument for Data, the Doc or the Hirogen holograms than there is for Moriarty. But that qualification certainly doesn't apply to just any random hologram.
This is pretty much how I see it. The same standard could be said to apply to droids in Star Wars, as some of them are clearly advanced enough in programming to meet the general qualifications for sentience (Threepio would be an example). Others are less advanced and suited primarily for whatever their basic programming is, such as power droids. One could modify them to be on the same level, but such a modification wouldn't be especially common.