I don't know for sure, but I'd guess that rolling back on a hill is probably the number one cause of concern for inexperienced drivers of manual transmission cars.
This is one thing that kept me away from them for a long time. I tried learning in my friend's Mustang GT, all mufflers deleted, straight pipes, at 3 in the morning (he worked really goofy hours and this was the only time I could get him to show me for awhile) in a QUIET neighborhood. I'd try to get the thing moving and kill it, then I'd have to restart and try again with the accompanying loud burble. I got so nervous I just gave him the keys and asked if we could just go to a flat spot so I could get better at that.
Hills are still hit and miss for me. I don't use the e-brake, but my car still shutters. My timing is still off a little. What freaks me out are the mega-steep hills where you can pick up a lot of momentum fairly quickly, in the wrong direction, and a tool who wants to kiss your bumper.
All that being said, haven't these been in all manual Subarus since at least the 80s?
I will have to say that I drove my friend's, same one who had the Mustang, new Jetta and it had HSA. It was the strangest thing ever. His car also has a vibrating clutch (not sure what that's all about - but that's another topic).
2013 Focus SE 5MT - Race Red (Clifford)
1998 Blazer LT 4AT - Black (Olie)
1998 Malibu 4AT - Powder Blue(The Costa Concordia)