Because hardware is NOT faster. Hardware RAID will allow you to abstract the OS and provide BB caching, as well as some proprietary RAID variants, but don't be fooled into thinking it's about speed gains. If you are serious about speed you would be looking at a pair of QLogic cards and a small SAN.
If you want to continue and buy a RAID card thats your choice. Just be clear in your understanding of what you are gaining.
We can debate/argue the point, in my experience hardware is faster, I haven't read that it's faster (obviously I have read articles, but this isn't my point) I have run both and found this to be the case. There are other reasons why hardware is better and you mention a few. But we can agree to disagree on it.
I am serious about speed to the point my wallet will allow, unfortunately it's on fire now as it is after buying the Mac Pro, 8 more gigs of memory, four V/Raptors and two 27" Dell monitors.
And soon, I hope, a decent RAID card.