The sound currently gives you very precise feedback of the state the engine is in, regarding load, etc. Once you learn a car, you can tell the shiftpoint just by listening to the engine note.
With a sampled sound this is not the case, or very very hard to achieve. The generated sound also works 100% correct if you're not giving full throttle, which is often a problem with games using sampled sound engines. GTR2 for example sounds good on full throttle or on no throttle, but if you're in between it simply sucks, and at times you can even hear the sound sample switching back and forth from "no throttle" to "full throttle". Or CSR, for example, sounds absolutely pants if you don't mix it with the LFS sounds.