I wanted to know if composing music for a specific instrument demands that you know how to at least play the instrument. I'm sure if you know how the instrument sounds by itself as well as within an orchestral setting that's enough. As a composer, should I be learning new instruments I want to compose for or that I will be composing a lot for? I feel like learning how to play an instrument should only be done if you would enjoy playing that instrument regardless of whether you compose for it or not? As a composer, it seems silly to force yourself to learn an instrument just because you think you can get a more intimate feel for the instrument in doing so.
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.
jjmusicnotes' answer is a good answer, and I'd like to make a special plea for the percussion section. I know many musicians don't have much time for percussionists, but as a composer the section can be your ally. It can keep the ensemble together (whether there's a conductor or not); it can give you a lot of support for dynamic changes; and it can help make tempo or time signature changes easier.
As a percussionist, I've often been given a part that makes no musical sense, or even one that isn't physically possible to play. Even good arrangers often fall into the trap of guessing what percussion instruments and style might give the sound they want, instead of indicating what sound they want and letting the player work out how best to achieve it. This is crucial outside of a pro orchestra context, because different groups will have different equipment available: there's no standard percussion section.
It's really easy for even an inexperienced player to spot a part that was written by someone with no percussion experience. You don't need to learn every percussion instrument to get it right, just speak with percussionists (in whatever ensemble you're composing for) and get them to show you a good part and a bad part.
You don't have to learn to play the instrument, but you should definately learn about the instrument and it's possibilities and limitations!
If you don't learn about the instrument you run the risk of
To learn about an instrument's possibilities and limitations you could for instance
First thing I literally HAVE to say: Learning an instrument you're writing for is not just for emotional/spiritual/warm-fuzzy-feeling reasons ("eg I have learned to play flute and now my flute and I are one, wandering the earth as band geek ascetics" [I do not play flute; that was just an example]) --there are many practical and educational reasons behind doing so. Not only does knowledge of the instrument you're writing for enable you to write higher-quality, more cohesive parts that can be played with less effort, the more ensemble settings you play in and the more well-rounded you become as a musician, the better you will be as a composer overall. (hopefully that made sense) And I have to ask: If learning a new instrument sounds so "silly" to you...what instrument do you play? (just wondering)
I have done arrangements (as in arrangements of songs) that involved instruments I didn't know. In many cases, learning the instrument just to be able to write for it is impractical, depending on the skill level of the target audience. Some situations and examples that come to mind, either from my own personal experience or just my general knowledge:
In the event that you do need/decide to write for an instrument you don't play [as others have said more concisely]:
That's about all I've got; the other replies cover the bases pretty decently. I wanted to put in my five billion bits though (I can never stop at just two). Happy composing!