On: Why are D-sharp and E-flat considered to be two di...

[thaumasiotes]

Well, no, that would only work if you were committed to never using notes that weren't in the key signature. That would be an unusual choice.

[iainmerrick → thaumasiotes]

Right, but if your tune mostly uses conventional major and minor scales (which most do!) you mostly won’t need accidentals.

Also, you’ll be able to transpose to any other key just by shifting the letter names up or down and changing the key signature. That’s a really interesting and useful property.

Also also, the notes with accidentals won’t change when you transpose (although the accidentals themselves will need rewriting).

Transposing music would be hellish without this system!

[im3w1l → thaumasiotes]

Not having to use accidentals for in-key notes still reduces the needed number substantially.

[PeterisP → thaumasiotes]

Depends on the music genre probably, there are oh so many song arrangements that never use notes outside of the key signature.