I had a loaner F30 last week and it had the stock system. I'm sure you could make do with it but for me it was not up to scratch. It didn't seem to have the right balance, a bit tinny rather than the refinement that the HK brings.
I like with the HK you can adjust the equaliser to get the balance you want. The sound seems to fill the car and more importantly drown out my less than perfect voice
Maybe I'm just spoilt now after having heard the HK, I may not be an expert on this but I think it was money worth spending. If you're going to be in the car listening to your own music a lot then it's worth the extra. If you're going to be listening to the radio more then perhaps you can do without