Our endurance oriented clients certainly have benefited from both strength training (SQT, DL, Press) and from some smart CF style mixed modal training.
It seem like the stronger the person is the faster they run or bike...swimming has not shown as linear a correlation.
The literature indicates better inter and intra muscular coordination (strength training) benefits all of these endurance activities. If one can fuel a given movement with say stronger, better coordinated type 1 fibers this will be more efficient than relying on more fatigable type 2A's. Makes sense.
If someone were say....racing road bikes in Europe for a portion of the year the months leading up to race preparation might include some dedicated strength work 2-3x per week for the big movements...perhaps some metabolic conditioning. Once the season is rolling one need only hit those big lifts (sqt, dl, press, pull) 1x/7-10 days in something like an 8x3 or 10x2 format to maintain ~90% of peak strength. That should involve a minimal time investment with good return and no degradation of recovery ability.
"Survival will be neither to the strongest of the species, nor to the most intelligent, but to those most adaptable to change."