While piano is my main instrument, I have studied many others. Some I have dabbled in, some taken more serious study. Organ and clarinet are my main secondary instruments.
I've also studied under teachers that were multi-instrumentalists and those who were mono-instrumentalists. In almost every case the multi-instrumentalist teachers had a much more rounded education, and had a better understanding and appreciation of varied perspectives. It is one thing to study an instrument on paper, like in an orchestration course. It is a complete different concept to physically touch an instrument, learn to play it, and express music with it.
Piano has some peculiar quirks or "pianisms" particular to the expression of music on the instrument. Some of these are so heavily engrained in the canon of piano teaching that it often goes unsaid that such methods are specific to the piano, and may not be particular to every instrument or ensemble.
I've found some piano-only pianists, find it hard to work with methods of musical expression that differ than that used on the piano because they have not made the effort to understand the alternate perspective of the other instruments.
Piano is very successful at being a solo instrument, and the nature of that beast can also be its downfall. Many orchestral players take up a second instrument as an accepted part in studying their craft. They learn early on how to navigate the differences and similarities of instruments and instrument families.
Do I think studying other instruments is helpful in teaching? Without a doubt, absolutely so!