kenshinvt

It is true that it is small and lightweight. However, here are some reasons why I don't think it is worthwhile:
- it is only a permanent repair if you damage exactly one link and the surrounding links are in acceptable condition to accept the replacement link.
- it's not significantly more effort to wait and push the link back out and repair it once home, when you should reinspect the chain for the extent of the damage regardless of how you choose to repair it on the road. Running one or two links short does also not cause a significant inconvenience while riding.
- there is no such thing as a universal master link. master links are different across brands. Even within a brand, they differ across chain widths. Then when you get into some brands (e.g., Shimano), you also introduce other necessities such as installation pins. This means that your repair kit is bike-specific unless you have the exact same chain on every bike or carry multiple links at once.
- however small, it is yet one more thing to carry

not to rant, but I try to carry as little as possible with me. I haven't seen a persuasive argument for it, beyond perhaps if you are touring (which changes everything).