feline1 wrote:But let's be clear: if you are using an instrument that's stuck in equal temperament, it really is meaningless to worry about which enharmonic equivalent note-name to use. In reality, you cannot have either of them!
I disagree.
The point about using the correct note-names for a chord rather than their enharmonic equivalents is not just that they sound different (in which respect you're right - on equal tempered instruments they don't). It's that they make more sense, making the tonal and harmonic structure of the music clear to the person reading it.
This operates firstly in respect of individual chords. In most kinds of harmony in western music these are built up in 3rds. If you're using that kind of harmony, and you keep returning to say a dominant 7th chord G-B-D-F, then on one occasion you want to alter it by flattening the fifth, it makes sense to write it G-B-Db-F. The eye immediately sees that it is the same chord, but with a flat in front of it - the immediate visual impression matches the aural impression. If on the other hand you were to write G-B-C#-F, then the eye would be immediately thrown by the second from B-C# and the fourth from C#-F, and think it's some kind of inversion of something else.
Secondly, the correct spelling of accidentals gives a clearer impression of the sense of voice leading and progession from one chord to the next. Say in the example above, one wanted to resolve the chromatic note onto the diatonic one, rather than sustaining it for the full duration of the chord, then it WOULD make sense to use the C#, thus:
F.........
C#...D....
B.........
G.........
A musician's eye tend to read in "chunks", so would still see the overall sense of a G7 chord, and the visual upward movement from the C# to the D would, again, reflect what the ear hears. It's also important that in this case the D belongs to the chord whereas the C# is foreign to it - so it makes sense that the "look" of the chord is made to add up only at the resolution.
You may think this is pedantry, but I can tell you that these things make a difference for example to a pianist or keyboard player sight reading a piece in a session or trying to learn and understand a piece in depth (or even to players of purely melodic instruments, in some cases). For an orchestral score, they are absolutely essential to allow the conductor to follow and internally hear the harmony easily.
One can always say that technically it doesn't make any difference, but music is not an abstract technical science. It's something that interacts with human psychology, in real time. In this respect it certainly does make a difference.
Finally, if someone is composing music in the classical way, actually writing it down and giving it to musicians to play, then I'd say there's a real problem from the composer's OWN pont of view if they don't see any difference in writing a C# or a Db. It means they don't have sufficient awareness of what that note is doing in relation to the other notes of the composition - what function it serves within its chord, and where it is going in the voice leading. Answering those questions and answering how to spell the note go hand in hand.