I'll discount the grep example too because, as you say, it'd be borderline stupid to keep something like source code on a RAM disk. And before someone suggests it, if you've copied it there from a fixed disk first you've already paid the price that getting it into the filesystem cache would require.
Actually, I'd not be sure.
Application compilation is often done in multi-pass way. If the project folder is big enough, it would be very likely to go beyond the filesystem cache size.(That's why I see compile time improvement by copying the project folder to a ramdisk first)
Also, filesystem cache manager handles read operation with "sequential read" hint and "random access" hint differently. (See Raymond Chen's blog two weeks ago) Random access opeations do not enjoy prefetch in the current implementation.