As a composer, should I be composing music for an instrument I don't play?

As a composer, you mostly end up writing for instruments that you yourself don't play. Apart from Hindemith, it is fairly impossible to maintain a high level of proficiency on every instrument - there just isn't that much time and it is not feasible.

However, that does not excuse having a working knowledge of the instrument. Though it might seem silly to you, learning the rudiments of each instrument is hugely beneficial to aiding your writing. By physically working with each instrument, you gain insight into things that it does / doesn't do well, sounds it makes / doesn't make, and considerations specific to performers (such as allowing enough time for mute changes or picking up mallets.)

Even if you just squawk your way through Book 1 of each instrument, you will thank yourself many times over for your personal investment. I speak from personal experience. It was easy for me since all of the instrumental pedagogy courses were built into my degree, so by the nature of what I did I ended up learning how to play all of the instruments on a rudimentary level. This knowledge has greatly informed my writing - not only writing idiomatically but also with phrasing, texture, and orchestration as well.

It is perfectly acceptable to write music for an instrument that you don't know how to play - all that matters is that you know how to write intelligently for the instrument, how the instrument behaves, and how to make it sound good. It is instantly apparent when a composer does not know how to write for a given instrument, and all that does is make the composer look very novice - since they in fact are.

It is not enough to understand how an instrument functions soloistically or within an orchestra - it is also paramount to understand how that instrument functions in chamber settings, in choral settings, and with different families of instruments. I would write for two clarinets very differently than I would write for clarinet and trombone duet.

As a composer, you have nothing to lose by learning how to play an instrument - if for no other reason than doing so will help make you an informed composer and therefore make your music better. Wouldn't you want to write better music?

That said, I realize that it is not always feasible to do so (though it is quite affordable to rent an instrument for a month or two with a method book.) But in addition to learning what you can, I would highly suggest studying texts on orchestration. Walter Piston, Alfred Blatter, and Samuel Adler have all put out excellent texts on the subject. In addition, I would also highly, highly recommend that you talk to people who play the instruments for which you're writing. They've already put in all of the time learning the instrument and can offer invaluable insight. You can have them play through sketches and determine what works / what doesn't work. In addition, it serves as a great resource for networking, which is crucial for getting things performed.

For composers, getting things performed is the bottom line.

Hope that helps.