Historically, there used to be a huge advantage with SCSI.
You could connect many more devices to a SCSI controller while only using a single IRQ, whereas IDE controllers required 1 IRQ per independently accessible device. This was important when you only had half a dozen IRQ's to divide amongst all your peripherals.
SCSI devices incurred much less CPU utilization before the advent of DMA
SCSI devices were often better performing, both with optical drives and hard drives
SCSI hard drives were available with higher spindle speeds and faster actuators
Today, IDE has managed to make some of these points moot (see below), but old habits die hard.
ACPI/APIC allows IRQ sharing and virtual IRQ's.
SCSI devices still incur less CPU utilization than UDMA, but the difference is smaller, and CPU's are so powerful these days
SCSI devices are still better performing, but for hard drives only, and IDE drives have improved performance on the desktop
SCSI hard drives are available with higher spindle speeds and faster actuators, but the large caches with large caches and optimized firmware of today's top IDE drives can mitigate the performance cost of inferior mechanics in some (but not all) situations
I have always been a strong proponent of access time being one of the biggest performance factors when it comes to desktop usage (despite the fact that large caches and optimized firmware in IDE drives have been able to mitigate the impact of slower access times in some, but not all, situations) and still say 15k SCSI is a nice thing to have if you can afford it.
However, there is a new class of drive in town: the 10k SATA Raptor. This is the new power user drive. I would say this is a better choice for most power users over both SCSI and IDE drives.
I have been using SCSI for ages for some of the same reasons .... and a desktop is much more "awake" with a 15K SCSI disk. I don't know if my usage is different from most people, but I constantly change between program and data, expecting that my harddisk is used to read from all sectors at the same time
I have no doubt that many people will never see a performance benefit from using a 15K SCSI drive instead of a SATA disk. My girlfriend has an IDE disk but otherwise the systems are very much the same, P4 2.5 Ghz, but my system is more awake.
I have considered using SATA Raid for my home server instead of SCSI, but a SATA Raid adapter that support Raid 5 and perhaps 6 disks is very pricy and the products are still very "young" - I still need to see IBM etc. supporting SATA RAID.