Actually right now I personally have the boot, applications, fast project i/o, and scratch all set to use the same 3-drive RAID 0 and I'm using the other 1TB drive for slow storage of like iTunes and E-Books and stuff. I set it up this way a few months ago as an experiment to see if there were actually any performance hits like everyone says. So far there aren't any differences at all other than that the boot is faster and app loading and searching is quicker. So I'm kinda thinking at least for my usage style, that such are wives tales.
But what I was saying above is like you said. This is how I had it for over a year (prior to my little experiment) and meets with most people's approval as discussed in these kinds of forums.
I'm with you... the idea of splitting storage functions across multiple physical drives is old-school. The reason that practice started in the first place was to try and minimize latency hits from drive heads sweeping across the platters doing a variety of things at once. Now with cache sizes and NCQ and particularly SSD's with near-zero latency this is all moot.