Most brands have 15k drives but only SCSI, fiber channel and SAS
and yes, they would be very good for a scratch disk, but I would also have my OS disk being a 15k.
However, you are paying a premium for that last 3-5%, but 3-5% improvements in all the bottlenecks helps
I currently have a setup like this, with OS, scratch and windows pagefile all on seperate 15k disks and data stored on 10k 300Gb disks all SCSI U320 havent moved to SAS yet.
Henrik