DanielB said
Well, if you are gigging or jamming, or paying for recording sessions, then I'd say 2 is more like the minimum.  If you break a string or the tuning suddenly goes wonky, you don't want everybody to have to stop while you take care of it.  Just switch to your tuned and waiting backup at the first sign of trouble. 

But beyond that, if you work in alternate tunings regularly, then "extra" instruments definitely do not have to just gather dust. 

It can be argued that an absolute beginner has enough to worry about at first without dealing with things like alternate tunings, but even fairly early on a song is likely to catch your fancy that just happens to usually be played "funny tuned".  I don't think it is that big a jump after a player has made their first few months of progress, and is comfortable with the idea of a special tuning being "normal" for certain songs/sounds.  It does mean a bit more practice, since alternate tuning take some practice on their own, but a lot of players are up for that.

I don't have that luxury with violin (YET!  LOL), but for example I have two 6 string acoustic guitars, and I usually keep one in standard tuning and one in whichever alternate tuning I'm working with this week.  I also have two electric bass guitars, and I keep one of them tuned standard and one tuned to what some metal players call "slack B" tuning.  That goes way down in pitch; I think the orchestral stringed instrument in that range would be the extended (5 string) contrabass.  I don't think there's many orchestral pieces that actually require that range, but it is not unusual in old jazz, rockabilly or metal, and it is just fun to play that far down in the bottom sometimes.  It takes heavier gauge strings and a higher action to keep them from buzzing when you tune a bass guitar that low, so it could not be done well by just retuning the regular bass for a song or two. 

Sometimes instruments also have sentimental value for us, and so we may have more than someone else thinks is "enough".  Like with Coolpinkone's "pinky", it was a gift bought for her.  So just selling it off for whatever she could get for it might not feel right.  On the other hand, I know she has at least thought hard about re-stringing "pinky" to tune down to viola range, so she can give that sound a try.

But if one does end up with instruments that are truly "extra" to the point of being redundant and gathering dust, there's always the possibility of gifting them to a friend or family member who want to get started learning.  Local schools and college music departments also are usually very happy to get an instrument donated to them.  Even ones of very humble price-tag origins, if they are playable or can be made so with some work.

I am in the market here soon for a 2nd Chinese Fiddle for exactly alot of the reasons that you mentioned Daniel. I too experimented with a few alternate tunings....I can't remember which.

But in my case I have to becareful because it's a slippery slope. Whenever I've had more than 2, it always turns about to be the 1 or 2 that I play exclusively while the rest just become "dust bunny collectors" that sit around or I end up selling.

Since I've been in Argentina I've given away a few instruments and sold a few others ( 2 Banjos I just recently sold.

Since I've been playing fiddle it takes so much time away from my other instruments I'm now down to just "1" Banjo and "1" Mandolin  ( 1 fiddle, soon a 2nd....for good....which will do it for me.

I'm looking for a 2nd fiddle that's going to be cheap and inexpensive enough that I won't freak out if it get's stolen or if I take it out to a bluegrass jam in a field or in the woods somewhere. But I want it to be loud, bright and raunchy sounding enough that it will stick out in a jam. My Gliga is a great around the house fiddle or maybe for a small Bluegrass ensemble but it's a bit too smoothe sounding for a rambunctious Bluegrass jam at a festival or Wal-Mart parking lot LOL. Even so, I won't be back in the US until later this year and I'm not ready to hit the jams yet so I've got some time. Yet, I've got my eye open either way. Also if I want to experiment with some alt tunings and stuff too like you mentioned.

I've got my eyeball on some decent Chinese fiddles....

My wife would say that everything I've just written is not true hahahahah!! I'm passionate about music and I love toys as much as the next guy...so who am I kidding right!?