Recently learned on a tour with broken derailleur for several days (removed) :
- carrying a half link compatible with your chain can really help you find a suitable gear ratio for your non-horizontal dropout frame if you have a derailleur or hanger problem. This 3 grams could save you hours.
- when field replacing a derailleur that might have damaged your hanger, thread it from the inside first [if possible - sometimes your bolt may be removable to do such or try inverting the derailleur ] to chase the threads prior to trying on the outside of frame. The stiff bolt (often steel) will give you a much better chance of being able to thread the new derailleur on instead of never being able to get it attached again. You're glad you brought an adjustable wrench now to straighten that hanger? Needless to say, if your hanger was aluminum, hopefully you brought an extra, or one of the "emergency" hangers that can be clamped onto with your axle quick release.
My situation : mudded trail destroyed rear derailleur, thrown into wheel. So removed it and went single speed. However, couldn't find a suitable gear where chain wasn't bouncing everywhere. Quite often the chain could sit in a "rough combination" (best chain length for that combination, but not tight enough tension to not jump over cassette cog teeth), but any bounce, change in tension, speed, out of seat climbing, etc. would cause it to fall. Falling could go to lower or higher gear, going to lower gear was very hard to get chain out of because of super chain tension that could have eventually caused BB/rear hub bearing failure (which I thought would happen but didn't). The half-link would probably allowed me to sit in the gear I wanted to be in without bounce, or at least less. I ended up pedaling about 100k over 2 days on single track before being able to replace derailleur.
Need for this is dependent on your gear ratios/chain stay length. Might be a good idea to think about this for planning - look for "Magic Chain" or Magic Ratio links as some frame setups are ideally suited for this, or to understand where your "emergency single speed" gearing will put your chain tension compared to normal chain elongation.