I live in New brunswick canada. Just about every day, I will get a car in the shop for brake lines because they rusted out. Most of the time, the lines just blow out, and then the customer realizes all of his/her lines are rusty (or some) and need to be replaced.
I see what you're saying about lines looking rusty and still holding up well, and that is true if its just surface rust. However, if there are little chunks of rust on the lines that look like they will break off, i'd replace the lines.
IN N.B, we have saftey inspections once a year, and we fail brake lines if we question their saftiness on the roads, especially with the nasty winters and the amount of salt on the roads. All it takes is a panic brake, a line bursting, for an accident to happen, especially with inexperienced drivers. Its all about using your judgement to determine the condition of the lines.
However, having the car undercoated, including a thorough coating of the lines, and those brake lines will last a long time.
2009 Pontiac Vibe AWD 2.4
1993 Buick Park Avenue 3800
1984 Chevrolet Citation 2.5 liter four.