Why do people carry extra bolts and firing pins?  Do they break often?  I notice lots of accessories make room to house extra bolts...

Although it doesn't happen often, firing pins and bolts can break.  If you are in a combat situation (i.e. Iraq) it probably wouldn't be a bad idea to carry a spare.  You know, just in case.  Your life may depend on it.  

Quoted: Although it doesn't happen often, firing pins and bolts can break.  If you are in a combat situation (i.e. Iraq) it probably wouldn't be a bad idea to carry a spare.  You know, just in case.  Your life may depend on it.

+1


Hooah!

it would also ruin a fun day if it were to happen and you didnt have a spare

But  just to give you a bit of reassurance, the Colt SP1 carbine I've owned since 1978, and fired multi thousands of rounds thru, has needed only one part replacement in all those years---a new extractor spring

Yes, I keep a spare firing pin around for my 5 (soon to be 6) ARs, but I've never had to use it.  What used to be my spare bolt assy, has gone into the build I'm in the middle of.  If I luck onto another milsurp Colt bolt at a good price, I'll probably buy another; but in the meantime I'm not particularly worried.

ARs, when well built  by one of the major outfits, are reliable and long lasting.