I'm quite sure that HP would be happy to sell you a 3of9 barcode on a DIMM.
If you are only using one printer it's not too bad, but if you are printing barcodes on a large number of printers, then it can get rather pricey to put a font DIMM in each of them and a soft font can look pretty attractive.
Again, it all gets back to what PCL command the application uses to select the barcode font.