What is silly about it? I wasn't talking about using a piano to figure out things. I'm talking about having a piano score in this case that was written without any instruments other than piano in mind. Then afterwards looking at the score and saying - this would work for violins, this would be good for flute, etc.
In the case of Mahler, you do not have that situation (at least a lot of the time). He probably originally had in mind the sound of a trombone solo in the third symphony, or a trumpet solo in the fifth for example, that was completely integrated into the musical idea to begin with - was not separable from it.
An example of the opposite is Bach - the Art of Fugue. There are no instruments at all specified. It is pure musical tones. Yet you can after the fact orchestrate these any way you want. These are two extremes - two distinctions - and there is nothing "silly" about them.
In the case of Mahler, you do not have that situation (at least a lot of the time). He probably originally had in mind the sound of a trombone solo in the third symphony, or a trumpet solo in the fifth for example, that was completely integrated into the musical idea to begin with - was not separable from it.
An example of the opposite is Bach - the Art of Fugue. There are no instruments at all specified. It is pure musical tones. Yet you can after the fact orchestrate these any way you want. These are two extremes - two distinctions - and there is nothing "silly" about them.