Just a few thoughts.
1) High reps are good for restoration both to let you train harder again sooner, and for causing growth.
2) ENDURANCE in general will help you get more out of your workout, but I don't think high reps is the way to go. HIIT will help you pack more hard sets into the same time frame. I really believe that you should either A) train hard for not more than 45 minutes or B) Train your as off for no more than 25 minutes. HIIT will help with this.
3) I think that "strength endurance" is best built with heavy weights for low reps but with short rest periods rather than training for high reps. 10 x 3 at around 75% in 10 minutes being a good example.
When would I use low load "high" reps (around 12-20)?
1) After heavier work on the same muscle group.
2) For a muscle that is relatively undeveloped compared to the rest of your muscles. For example, if you have weak calves, external rotators or forearms.
But at any rate, I think its still better to do say 5 x 10 with short rest than 20-30 reps continuously, even if the 5 x 10 are not challenging.
Finally, Local endurance strength does help more complex endurance activities. High reps for hip flexors and shoulders will transfer to say running a mile if that's your goal, but I still think several heavier sets of 3-5 with 1 minute or less rest periods works better.
And again, I'm just thinking out loud here.