Best kept secrets in tele lenses?

Not necessarily so. I know people who's been using manual-focus 300 to shoot birds (without focus peaking). Not easy but possible, and likely would be missing a lot of opportunities that a fast AF camera would handle easily. Birds come with different sizes. Small birds move faster and move a lot even when perching. Larger birds, especially large ones such as blue herons or even eagles and owls, not so much and slower. So at the end of the day, you can shoot bird photos with MF lenses but just not of any birds. Shoot with good timing, pre-focusing and the appropriate use of depth-of-field are some of the ways to take action shots with manual focus lenses (even in the old, film days).