8+ speed chains are riveted rather than pinned, and when a rivet comes out -- either accidentally on the road or pushed out by your chain tool -- the flange on the end of the rivet is broken and will no longer hold reliably. As you note, such repairs do not necessarily fail immediately, but they virtually ensure an eventual failure at the link. If you use your chain tool to emergently repair a chain on the road, take it easy until you get home and then replace the chain as soon as possible.