You're right, quoted seek times don't always mean much, but compare the quantum to any ATA drive out there, it'll beat it out. (I own the Atlas V, BTW).
CPU% per IO is actually the most useful gauge for finding out what sort of CPU utilization you're really going to get. That way you can calculate how much CPU utilization you'll get doing 100 (or however many your program is requiring...) IO/s. And the Quantum will win, thus, lowest CPU utilization.
"When was the last time you saw a software anything that was faster than hardware?"
Actually, in the tests with the promise RAID card, I believe software did come out on top, at least in terms of CPU utilization. Weird huh?
"The Promise controller is not a true hardware RAID implementation, proven by the fact the hardware is basically identical to the standard IDE controller made by Promise."
When people suggest an IDE raid setup this is the card they're usually recommending because it's cheap. If you move up to a better RAID card it becomes less cost effective as compared to going SCSI.
I have no idea what system setup you have, but I have never heard of anyone defragging a hard drive while burning a CD. I have a Seagate Cheetah X15 and a Plextor 8/20 CDR, and it wouldn't even cross my mind to do something that dumb.
There's nothing dumb about wanting to defrag your hard drive, it is beneficial for your computer. Really.

Here's my main system:
IBM 18ES LVD 7200 RPM
Quantum Atlas II SE 7500 RPM
Ultraplex 40MAX
Yamaha 4416S
all SCSI, obviously. I was actually running the defrag while burning to see just how much I could do while burning, before I made a coaster. And it worked just fine.
In case you're wondering where the Atlas V went, it's in my other system, which I haven't messed with as extensively:
Quantum Atlas V LVD 160 7200 RPM
Ultraplex 40MAX
Plexwriter 8/20
You are correct about another of IDE's faults, only one device may be accessed at a time. But try getting on an IDE system, start burning and do something very processor intensive that doesn't involve the hard drive much. won't work.
I can't run HDtach in Win2k unless I want to upgrade to the full version, but when I ran it in 95, my results were much better than yours.
HDtach reported 0% CPU utilization.
HDtach isn't the most reliable or accurate benchmark anyway.