Nature does not lump or split. The universe is a continuum, without the types of seams that splitting requires and with many more degrees of subtle variation for homogeneous lumping.

As a technique for understanding nature and the umniverse, both lumping and splitting have their purposes and can be useful - but only when applied properly. Both can be counter-productive when taken to the extreme, or adhered to when situations would dictate otherwise. Making distinctions is what allows doctors to figure out which ointment to prescribe, or which part to lop off, or which artery to unclog. In fact, it is our ability to make finer and finer distinctions - to measure things ever more closely - that keeps moving the species forward, technologically speaking.

The thing is to be able to see the differences in like things and the commonality in disparate things. And the trick is to alter your focus. It's like the old zen riddle says: First there is a mountain, then there is no mountain, then there is. Or was that Donovan?

This all applies when talking about music. I think in music, a single-minded obsession with categorization and definition is just a step on the road to madness. To say that a Tuba is not a bluegrass instrument is disingeneous; the truth is probably closer to "I don't think I like the sound of Tubas in a bluegrass band". To say that in your experience, Tuba hasn't worked well in a bluegrass context might be closer to the truth. To listen to the Tuba player play "Foggy Mountain Breakdown" before making a judgment about whether she/he adds to the music, is to hear bluegrass again, for the first time.