Why do conductors use transposed scores vs. concert scores? It adds a lot more mental cost with no return on investment.
Why do conductors use transposed scores vs. concert scores?
They don't always. They use what's available. Some pieces are published as transposing scores and others as concert pitch scores. I'm not aware of any edition that offers both for the same piece.
It adds a lot more mental cost with no return on investment.
This is true for some purposes, but not for others. To relate one part to the rest of the orchestra, the reader of a transposing score has to transpose the written part to concert pitch, but to communicate with the player of a transposing instrument, the reader of a concert-pitch score has to transpose the written part to transposed pitch.
The two principal contexts in which the reader of a score communicates with the player of a part are in rehearsal and in copying the part. The latter is more likely to be automated in the last 30 years or so, but traditionally it would have been a more significant consideration: there's a greater potential for human error when a copyist has to transpose the part for a transposing instrument than there is when the copyist is simply copying.
In rehearsal, when (for example) the B-flat clarinet is playing the third of a D-major chord and the conductor wants to discuss that note with the player, there's actually less mental arithmetic needed if the conductor has a transposing score. The conductor sees a G♯ in the part and says to the player "can you play that G♯ a bit more quietly?" Simple. With a concert-pitch score, the conductor sees an F♯ in the score and has to transpose that to "G♯" before speaking to the player.