I disagree with your point.
I'm a part-time musician and my wife programs (selects music for) several classical ensembles.
It's very important that composers and arrangers understand a lot about the instruments for which they are composing, and even more so if they have to additionally understand educational requirements or the limitations of players beyond their instruments.
To do the work, you have to understand, for instance, the ranges of instruments you compose for, as well as the difficulties imposed by the mechanics of the instrument.
To be fair, folks like Mark Mothersbaugh can hand off their basic compositions to arrangers to implement, but this is not a normal situation. Rather, most composers, especially composers of pieces for educational ensembles like my wife's are largely judge by how playable a piece is-- this is a pragmatic limitation on how good a piece can sound.
I am a pop musician, and occasionally I run into pieces that have been composed for string ensembles by people with little knowledge of the instruments for which they are writing.
As a low-rent front end developer, these pieces feel _very_ much like the "web designs" that I occasionally implement for people who generally do print design: they are missing a lot of details that I have to generate in order to make them work on the web and there are often many misqueues as far as basic things like creating usable forms and buttons.
I don't believe that anyone would maintain that you can't create good work unless you have deep formal training in every aspect of a design.
However, I my suspicions about the abilities of, say, self-taught singer-songwriters whose only instrument is guitar and who can play barely enough keyboard to input a score into finale are often confirmed.
And that happens a little less frequently than my suspicions about print-trained "web designers" are confirmed.
