To me at least, hard edge brushes are way harder to use and they live little space for error, they also are quite hard to add line thickness and opacity variety. Soft edge brushes in other hand, are the oposite, they have a lot of opacity and size range + they have a nice texture to them. Think as if hard edge is closer to a pen in tradional midia and soft edge it's more like a pencil.
Of course that you can achieve some pretty nice line quality using the hard edge, but as i said, (i'm my opinion, btw), it is harder to use.
Here, i did a little example using both of photoshop standard round brushes, it's quite subtle, but there's a difference.
I dunno man, soft brushes just feeeel right to use.
